Explanation of Plate XI (figs. 1-15):

'Note. In the preparation of all the figures (except 15) Abbe's camera was employed and all, with the exception of two, are illustrated as they were seen with an immersion apochromatic objective (Zeiss 3 mm., 2 mm. or 1-5 mm. focus). The exceptions are Figs. 1 and 2, in the drawing of which Zeiss D. was used.

Fig. 1. Section of a villus from the pyloric end of the small intestine of a guinea-pig kept on ordinary diet. Alcohol, acid ferrocyanide mixture, balsam. x 305.

Fig. 2. Optical section of a slightly compressed villus from a guinea-pig after the administration of "peptonate" of iron. l, the lacteal vessel. Alcohol, ammonium sulphide, glycerine. x 305. 

Fig. 3. A portion of the mucosa of the intestine in a lake-lizard. e, epithelial cells, l, iron-carrying leucocytes, r, red blood corpuscles, also shown to contain inorganic iron. Alcohol, acid ferrocyanide mixture, balsam. x 620.

Fig. 4. A portion of the epithelium and underlying elements of an intestinal villus of a guinea-pig after the administration of "peptonate" of iron. l, leucocyte, bc, blood capillary. Alcohol, acid ferrocyanide mixture, balsam. x 1240. Drawn with the diaphragm of Abbe's condenser removed from the microscope.

Fig. 5. Portion of a section of the liver of a guinea-pig fed with "peptonate" of iron. l, leucocytes, hc, hepatic cells, bc, blood capillary. x 1240. Drawn with the diaphragm of the condenser removed.

Fig. 6. A portion of the tip of an intestinal villus of a guinea-pig kept on its ordinary diet, to show the distribution in the cells of the organic iron compounds (chromatins). e, epithelial cells, l, leucocytes, a, nuclei of adenoid elements. In the cytoplasm of two of the leucocytes are found granules of an inorganic (?) iron compound. Alcohol, nitric acid alcohol, acid ferrocyanide mixture, balsam. x 1240. Drawn with the diaphragm of the condenser removed.

Fig. 7. Epithelium and underlying elements from the side of a villus of a guinea-pig on the second day of the course of yolk-feeding, to show the distribution of organic iron compounds (chromatins). l, leucocytes, a, the sub-epithelial "membrane." Alcohol, nitric acid alcohol, acid ferrocyanide mixture, balsam. x 1240. Drawn with the diaphragm of the condenser removed.

Fig. 8. Epithelium and underlying elements of a villus of the same animal. a, sub-epithelial "membrane," s, the secretion from epithelial cells. Corrosive sublimate, haematoxylin, eosin, balsam. x 1240.

Fig. 9. Portion of a Lieberkuhnian gland of a guinea-pig to show the distribution of organic compounds of iron (chromatins) and especially of those connected with secretion. Alcohol, nitric acid alcohol, acid ferrocyanide mixture, balsam. x 620. Drawn with the diaphragm of the condenser removed.

Fig. 10. Portion of a fresh villus of a guinea-pig killed five hours after being fed with stale yolk. The epithelium has been removed. Ammonium hydrogen sulphide, glycerine. x 620. Fig. 11. Intestinal epithelial cell of a villus from the same animal. Alcohol, ammonium hydrogen sulphide, glycerine. x 620.

Fig. 11. Intestinal epithelial cell of a villus from the same animal. Alcohol, ammonium hydrogen sulphide, glycerine. x 820. (Zeiss oc. 4, apochr. imm. 1.5 mm.).

Fig. 12. Epithelium and underlying leucocytes of a villus from the same animal, to show the absorption of the yolk chromatin. Alcohol, ammonium hydrogen sulphide, glycerine. x 620.

Fig. 13. A liver cell of an Amblystoma fed artificially for four days with egg-yolk. Alcohol, sulphuric acid alcohol, acid ferrocyanide mixture, balsam. x 820. Drawn with the diaphragm of the condenser removed.

Fig. 14. Yolk spherules from a hard-boiled egg, to show the distribution of the iron (of the haematogen). In the spherule on the left the elements are fewer and coarser. Sulphuric acid alcohol (for 48 hrs.), ammonium hydrogen sulphide, glycerine. x 820. (Comp. oc. 4, imm. apochr. 1.5 mm.)

Fig. 15. Free border of an intestinal epithelial cell of an Amblystoma, fed with yolk. p, cell protoplasm, h, hyaline border, y, elements of yolk. Alcohol, ammonium hydrogen sulphide and glycerine (at 60° C. for eight hours). (Comp. oc. 4, imm. apochr. 1.5 mm.).' (295-297)

Fig. 4 in text:

'In very thin sections treated with the ferrocyanide mixture and mounted in balsam, the distribution of the iron was more clearly seen. Sometimes in the epithelial cells the blue reaction was a diffuse one with blue granules collected in groups here and there in the cell, in some instances it was fouind in the inner end of the cell chiefly, while again the protoplasmic processes in the hyaline border gave an intense reaction. Fig. 4 shows some of these details distinctly. In this are represented three cells, in two of which the inner ends appear loaded with iron and they were fixed in the act of transferring it to the underlying tissue.' (272)

Fig. 5 in text:

'When the dose of iron given was not great, then the iron was mainly if not wholly confined to the peripheral zone. With large doses a greater portion of the lobule was impregnated with iron, and other elements came prominently to view, especially in the "peptonate" preparations. There were leucocytes in the angles of the capillaries in all parts of the lobule but very frequently in the central portion, and their occurrence was manifested under the low power by the strong reaction which they gave for iron (fig. 5). Sometimes each cell was a mass of blue material or it contained large blue masses, in others again, the cytoplasm had a diffuse blue tint with one or more clumps of ironholding substance.' (273)

Figs. 6-8 in text:

'In the guinea-pig, as ordinarily fed, the "masked" iron exhibits in its distribution in the epithelium of the intestine very little difference from that represented in fig. 6, in which the iron is shown in the chromatin of the nuclei and in a narrow zone immediately about some of the nuclei, but in preparations from animals fed with yolk for two or three days, the epithelial cells situated on the sides of the villi and below the tips of the same have the iron distributed as represented in fig. 7, in which the whole of the protoplasm in the lower half of each cell and in the leucocytes below give a uniformly diffuse Prussian blue reaction. The epithelial cells at the tips of the villi are so much distorted by the fat present in them, that a division of each into an internal and external part is impossible, except in some cases where the absorption of fat has ceased to take place. In preparations stained with haematoxylin and eosin, the cells immediately below the tip give usually the appearance represented in fig. 8, but with this exception, that the bodies enclosed in cavities of the protoplasm in the external half of each cell shown in the figure are not present in the preparations from all the animals fed with egg-yolk.' (283-284)

Fig. 9 in text:

'Now as in the secreting cells of all sorts, and especially in those of the Lieberkühnian glands, secretory activity is associated with the presence of a chromatin in the part of the cell remote from the lumen (fig. 9), it might be urged that the increase of the "masked" iron in the inner ends of the superficial epithelial cells of a villus was due not to an iron compound absorbed, but to secretory activity bringing about an increase of the substance governing that process.' (284-285)

Fig. 11 in text:

'It may, in fact, be that the oval vesicles were not connected with absorption at all and that they were merely appearances in the rodlets, although, on this view, the green reaction in their envelopes would be difficult to explain. That the rodlets are not always simple structureless elements, I have, several times, found to be the case in preparations from the guinea-pig (fig. 11), in which each rodlet appeared to be a series of beadlets or granules.' (293)

Fig. 12 in text:

'There was evidence of the transference of yolk chromatin from the epithelial cells to the underlying elements, and this was found in those villi in which the process of fat absorption had not distorted the cells. Sections of these villi, obtained from material hardened in alcohol, when treated with ammonium hydrogen sulphide, gave preparations like that of which fig. 12 is an illustration. The inner portions of some of the cells at the extreme tip of a villus gave a faint greenish reaction immediately after the reagent was added, but in the corresponding portions of other epithelial cells the reaction was given also by granular elements lying between and among the fat droplets.' (290)

Fig. 14 in text:

'The failure of all these experiments led me to use a less abnormal kind of food; and, since, according to Miescher [note: 'Miescher. Hoppe-Seyler's med. chem. Unters. Pt. 4, p. 502, 1871.'], egg-yolk itself contains 1 to 1-5 per cent of nuclein (haematogen), that food substance appeared likely to yield the best results.

I used unboiled egg-yolk, for when egg-yolk is hard-boiled the yolk spherules become thereby fixed in form, and the chromatin-holding particles are set free only when the spherules are digested, but wben the yolk is administered fresh the spherules readily undergo fragmentation and the chromatin-holding particles are liberated and put in a form in which the epithelial cells, if they possess the power, can invaginate them. In the spherules the chromatin is partly in a granular form [note: 'Miescher (loc. cit.) localised the nuclein which he discovered in egg-yolk in the granules of the yellow yolk spherules.'] and, apparently, partly as envelope material to its fat globules, the latter varying in size and shading off into the small granules in such a way as to suggest that the latter are also fat globules of almost infinitesimal size surrounded by chromatin. Fig. 14 gives a representation of two yolk spherules which were fixed by heat and in which the iron, set free by sulphuric acid alcohol, was converted by ammonium sulphide into the sulphide. In it can be seen smaller and larger fat globules surrounded by an iron-holding envelope. The fat is, therefore, closely associated with the chromatin, and as we know the former is in some way absorbed by the intestinal epithelium, the conclusion did not appear to be a strained one that both constituents are absorbed together.' (282)

Fig. 15 in text:

'With 15 mm. apochromatic immersion (Zeiss) and compensation ocular 4, the vesicles could be seen connected by a grayish line. On the protoplasmic side of the margin were also minute vesicles with greenish envelopes, apparently of the same character as those in the striated border and the reticulated protoplasm itself had a slightly greenish tinge (fig. 15).' (292)

Explanation of Plate VII, figs. 1-9:

'Fig. 1. Diagram of the Cardiac apparatus and gills of Octopus, with the cardiac and respiratory nerves.
Vc. = Vena Cava. KY. = Kidney Vein. BH.= Branchial Heart. G. = Gill. LA. = Left auricle. RA. = Right auricle. V. = Ventricle. CA. = Cephalic Aorta. Ga. = Genital artery. Va. = Visceral artery. SF. = Veins from sides. P. = Pleural ganglia. S. = Stellate ganglia. Vg. = Visceral nerve. C. = 1st Cardiac Ganglion. Cr. = 2nd Cardiac Ganglion. B. = Branchial Ganglion.

The dotted lines indicate the parts covered by glands.

Fig. 2. a. Muscle fibre of the ventricle of Octopus. (x 500.) Alcohol and Hematoxylin.

Fig. 2. b. Portion of a muscle fibre of the ventricle of Sepiola. (x 500.) Gold chloride.

Fig. 3. a. and 3. b. Plasma cells from the julletion of the kidney and auricle of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 4. Ganglion-cell from the cerebral ganglia of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 5. Plasma cell from the cutis of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 6. Connective-tissue cells; a. from the auricle, b. from the cutis of Pterotrachea. (x 500.) Osmic acid and Picrocarmine.

Fig. 7. Diagram of the branchial and cardiac nerves of Aplysia.

LV., RV. = Left and Right Visceral nerves. Vg. = Visceral ganglion. Gen. = Genital nerve. O. = "Olfactory organ." rn. = Left branch of the nerve to the gill. nb. = Branch to the pericardium. G. = Gill. P. = Pericardium. A. = Auricle. V. = Ventricle. a. = Aorta.

Fig. 8. a. Plasma cell on auricular muscle of Helix.

Fig. 8. b. Plasma cell from the connective tissue membrane between the loops of the genital duct of Helix. (Zeiss. D. Oc. 4.)

Fig. 9. Diagram of the cardiac nerves of Helix.

SO. = Supracesophageal ganglia. Sub. O. = Subcesophageal ganglia. Lv. = Left visceral nerve. Os. = Ovisperm duct. K. = Kidney. P. = Pericardium. A. = Auricle. V. = Ventricle. a. = Aorta.' (338-339)

Fig. 1 in text:

'The species chosen as most convenient for study was the common Poulp, Octopus vulgaris, and to it the following pages will be almost exclusively confined.

The cardiac apparatus of Octopus (Fig. 1, Plate VII.) consists of a pre-branchial or venous, and a post-branchial or arterial section. The former collects the blood from the veins and drives it through the gills; the latter receives the aerated blood fromn the gills and propels it through the bodv. The former section may be divided into the following parts:-

Vena Cava.

Kidney Veinis.

Branchial Hearts.

The latter consists of :-

Auricles.

Ventricles.

The Vena Cava (Fig. 1. VC.) is a straight thlin-walled tube, which runs down the ventral side of the liver, parallel to the intestine, towards the heart.

At the level of the front end of the ventricle it divides into two branches, which being covered with glands considered to have a renal function, may be conveniently distinguished as the Kidney Veins (KV.)

Each Kidney Vein, curving out towards a gill, is continued into a bulged oval body known as the Branchial Heart (BH.), their cavities being separated by a pair of valves at the junction. The Branchial Hearts have been usually held to be muscular organs devoted to propelling the blood through the gills; but that this view is not entirely correct will be shewn by the description of their minute structure given below. From each Branchial Heart a thick-walled "branchial artery" leads to the gill (G). The efferent vessels of each gill form a number of veins which soon unite to form a tuLbular auricle (LA. RA.), thin-walled but stouter than the Vena Cava. The two auricles open into the medianly situated, fleshy, somewhat globular ventricle (V), from the cavity of which the blood is prevented from returning by a pair of valves at each auriculo-ventricuilar orifice. 

Three efferent vessels issue from the ventricle, the most important being the great cephalic aorta (CA.), which starting from the posterior dorsal right region of the ventricle curls round to run to the head. Medianly and posteriorly is given off a small genital artery (G a.); and from the anterior ventral edge a small bulb gives rise laterally to a fine artery to each auricle and to a somewlhat larger visceral artery (V a.) medianly to the intestine.' (263-264)

'The whole cardiac system is supplied by a pair of nerves from the pleural ganglia in thie head... Their relations to the neighbouring organs and directions for finding them have been accurately given by Fredericq... Their branches and ganglia need however a more detailed description (Fig. 1). On issuing from the skull, each nerve appears as a double cord from which various small branches are given off. The cords on either side end in a small ganglion lying under the liver, from which go nerves to the body wall and the columnar muscle inserted into the mantle, while a single main trunk is continued downwards towards the heart. Just in front of the auricle it dilates into a ganglion, which may be called the 1st Cardiac Ganglion (C). From this issue a fine nerve to the generative duct, a nerve which enters the auricle, gives off a branch there and then passes through to the ventricle, and lastly a stout nerve which runs dorsally to the auricle down to the branchial heart, where it is connected with a ganglion-the 2nd Cardiac Ganglion (Cr). From this ganglion go nerves to the substance of the branchial heart and a short way into the kidney vein, but the main trunk proceeds to the gill, at the base of which it expands into the Branchial Ganglion (B). No other ganglia are revealed by dissection on the visceral nerves or their branches...

From the pleural ganglia there also runs on either side a stout nerve ending in tlle stellate ganglion (s) and containing the motor fibres for the mantle; so that the gills and heart are connected by nervous structures with the motor organs of respiration.' (267-268)

Fig. 2 in text:

'The fibres of the ventricle are of an elongated fusiform shape, with long tails, but are shorter and thicker than those of the auricle. Like the others they have no sarcolemma. Thus far they resemble the ordinary smooth muscle cells of vertebrata, but in addition they possess a fine but regular transverse striation very like that of the branchial heart, the fibres of which they closely resemble. Careful focussing also shows in the fibre indications of a granular core of a different nature to the outer zone. Both of these features are best seen in osmic acid or alcohol preparations (Fig. 2a) [note: This axial column of apparently less differentiated substance has been noticed by most observers of molluscan muscles. Thus Ranvier (Traite technique d'Histologie, p. 851), figures a gold chloride preparation of the retractor muscle of Helix pomatia showing distinctly the "cordon protoplasmique"; Leydig and Kölliker saw it in the buccal muscles of Gasteropoda, and H. Muller and F. Boll in the branchial hearts of Cephalopoda. (See F. Boll, Arch. f. mikr. Anat. Bd. v. Suppl. Hft. p. 28).'].' (266)

'while the ventricular fibres of Eledone resemble those of Octopus, those of Sepia present a bolder striation, and in Sepiola the difference is still greater. In this animal the fibres are much larger, and the average distance between the striations is 3.3m.; the dark bands are very sharp, but narrow; and the margin of the fibre is bulged out opposite each broad clear disc (Fig. 2. b). The central core is much more distinct than in Octopus, occupying about one third of the diameter of the fibre, and staining very deeply with gold chloride, while the outer zone remains clear. It also shows a beaded outline corresponding with the striations. [note: 'Transverse striation has also been noticed - chiefly in hearts or the buccal mass - by many observers, a list of whom is given by Boll (loc. cit.) and by F. Darwin (Journ. of Anat. a. Phys. Vol. x. Part Ii. April 1876, p. 506). Dogiel (Arch. f. mikr. Anat. Bd. 14) has also seen it in various hearts, (Pecten, Anodon, Aplysia, Helix) and Haller (Gegenbaur's Jahrbuch, 1883) in the hearts of Fissurella and Haliotis. Margo (Wien. Sitzusngsb, Bd. 39) has described the black bands in the striated shell muscle of Anodon as doubly refracting.'].' (266 [note 266-267])

Figs. 3-5 in text:

'Two kinds of elements however occur [in Pterotrachea] which might be mistaken for nerve cells, and which I believe have been taken for such in other Molluscs. The first of these are large, moderately granular, often roundish or oval cells, with a not very distinct oval nucleus placed near the periphery of the cell, and in which a nucleolus is sometimes visible. A distinct capsule is not present. These cells occur in great abundance on the walls of the rhythmically contractile kidney, and a few are usually to be found in the auricle [note: 'Although in teased preparations of auricle these cells were often found, I am inclined to doubt whether they occur normally in the auricle itself, and to think that in these cases they have appeared from adhering fragments of the closely connected kidney, which owing to the transparency of the tissues it is difficult to cut clearly off at the boundary. In preparations where the auricle was cut through at a distance from the kidney no such cells were found. At the same time it is possible that they may occasionally wander from the kidney to the auricle. They are also found in the pericardial wall.']. I have not observed them in the ventricle. Although the approximately oval form (Fig. 3 (a) Pl.VII.) is perhaps the commonest shape, yet it is by no means constant. In many cells short blunt processes or pseudo-podia are seen (b) which suggest the power of amoeboid movement, and occasionally a single such process may occur, causing some resemblance to a ganglion cell with the stump of a nerve fibre. The number and variations of the processes however oppose this idea, and a comparison of such a cell with a true ganglion cell (Fig. 4) at once shews a number of differences in appearance. Further these cells are never found in connection by fibres with either nerve or muscle, and are shewn to be very loosely applied to the tissues by the fact that in teased preparations large numbers of them become entirely detached and float like the blood corpuscles in the fluid in which they are mounted. Lastly, they occur not only in the parts mentioned, but in the connective tissue all over the body, as is well seen in preparations of the cutis, where they (Fig. 5) abound. These facts point to the conclusion that they are wandering connective tissue cells; and we may almost certainly identify them with the "plasma cells" which have been shewn by Brock [note: 'J. Brock. " Untersuch. ii. d. interstitiellen Bindesubstafiz der Mollusken." Zeit. f. Wios. Zool. Bd. 39. 1883.'] to be so characteristic of the connective tissue of some other Molluscs (Aplysiadae, Pulmonatae).' (321-322)

Fig. 6 in text:

'The second form of element which might be considered nervous [in Pterotrachea] is of much smaller size and more delicate structure. These cells again I found only where connective tissue was abundant. They are granular branched cells with a usually indistinct nucleus. Occasionally the branches may be reduced to two, and there then appears some resemblance to a small bipolar ganglion cell (Fig. 6 (a)). These cells however are nothing more than the ordinary connective tissue cells, which give off processes to form fibres and are found everywhere throughout the connective tissue (Fig. 6 (b)). A comparison of the two figures makes plain the identity of those of the heart with those of the cutis. These also are described and figured by Brock in the connective tissue of Aplysia as stellate connective tissue cells, and very similar cells by Haller as occurring 'in small quantity in the auricle of Fissurella and as forming a second type of ganglion cell.' (323)

Fig. 7 in text:

'Apart from the improbability of the existence of apolar ganglion-cells in the heart, the incorrectness of Dogiel's drawing of the heart and the adjacent nerves causes great doubt as to the accuracy of his interpretation of these cells.

In his figure (Taf. V. a. Fig. 15) of the "branchial" ("visceral," Spengel) ganalion he entirely overlooks the olfactory organ to which the right nerve runs, and he represents the left nerve as of equal thickness and as running straight to the auricle. But instead of such a short thick left nerve, there come off from the left half of the double "visceral " ganglion two distinct long nerves. Of these one supplies the generative duct, while the other runs on to the gill. Here it divides into two, the left one running on straight towards the hind end of the gill while the right curls round anteriorly and appears to end in the pericardium near the origin of the auricle (Fig. 7).' (325)

Fig. 8 in text:

'In the auricle of the Snail may be found upon the muscle bundles [note: 'Both the auricle and ventricle are formed of a meshwork of bundles of striated muscle fibres, that of the ventricle being denser and thicker.'] a number of plasma cells which are, there is little doubt, identical with Dogiel's "apolar!" ganglion cells. Some of these cells of an oval or pear-shaped form present a considerable resemblance to ganglion cells, and if only these were observed might be considered such. But when others of varying shapes, and others in process of division are seen, such an idea becomes untenable; and when the striking similarity of these cells with the plasma cells of the connective tissue is noticed, no doubt can remain of the identity of the two (Fig. 8).' The various stages of division also which are met with, from a group of two or three contiguous cells to a nest of closely packed small ones, point clearly to the true nature of these elements. True ganglion cells I believe do not exist in the Snail's heart.' (327)

Fig. 9 in text:

'The heart... receives nerves both at the auricular and aortic ends [note: 'A similar double nerve supply has been described by Haller, loc. cit. as existing in Muricidae and Fissurella.']. (Fig. 9.) The discovery of this nerve to the heart at once necessitated a reconsideration of the results obtained by previous workers, and the question of its function became important.' (328)