with its fellow to form a median organ. Its lower end then gives off solid, bud-like excrescences, and lateral buds come off again from these, so that this part of the organ acquires a ramified, lobulated appearance like an acinous gland. The acini are, however, solid, and remain so, although the upper end of the tube still has a narrow lumen.

The lymphoid cells next invade the epithelium, growing into every part of the tubular gland, and converting it into a mass of adenoid tissue. In this process the epithelium becomes broken up into small isolated portions, some of which remain in the medullary portion of the lobules as the epithelial nests which are seen in sections of the fully developed organ, and are known as the concentric corpuscles of Hassall. The liver. This organ arises in the form of two diverticula of hypoblast, which grow from the ventral wall of the duodenum immediately beyond the stomach (figs. 117, 118, 7, 123, Lb). They extend into a mass of mesoblastic tissue which connects the stomach and duodenum with the anterior wall of the abdomen, and which (with the mesentery, with which it is continuous round the gut) separates the body-cavity here into a right and left half. In this tissue is the omphalomeseraic or vitelline vein (and later the umbilical vein) proceeding on either side to the sinus venosus, and the liver diverticula grow into the mesoblast above and in front of these veins. Here they ramify, giving off solid buds of cells which grow into columns or cylinders, and these again give off lateral diverticula of the same kind. So far the development of the liver resembles that of a compound tubular or acino-tubular gland, except that the ramifications of the original gland diverticula are from the first solid instead of hollow. But soon an important difference appears in the fact that the cylinders unite and anastomose with one another everywhere to form a close network, and from the cords of this network solid sprouts are again constantly being given off to form fresh cylinders, thus producing a yet closer and more intricate network. In the meantime, capillary blood-vessels are formed in the mesoblastic tissue in which this formation of cell-cylinders of hypoblast is going on, and these vessels, which form a network interlocking with that of the anastomosing cell-cylinders, become connected with branches of the vitelline vein on the one hand (venæ advehentes), and on the other with veins (vena revehentes) which pass towards the sinus venosus, and eventually are found opening as the hepatic veins into the inferior vena cava.

The two original hollow diverticula are the rudiments of the right and left hepatic ducts. The common bile duct is formed later by a protrusion of that part of the duodenal wall with which the original diverticula are connected. This protrusion also eventually receives the duct of the pancreas, which becomes shifted towards it. As the common bile duct lengthens, the liver becomes separated from the duodenum, with which it was at first in close connection. The portal and interlobular bile ducts are formed by the hollowing out of some of the anastomosing cell-cylinders, so that a lumen is produced within them surrounded by hepatic cells, which lose their original polyhedral character, and become changed into the columnar epithelium of the ducts, the anastomoses between the cell-cylinders here disappearing. The remaining cylinders form the secreting substance of the liver. The biliary canaliculi appear as minute passages between the cells, and come into continuity with the bile ducts. With a further development of the connective tissue of the organ, the glandular substance of the liver, which was at first continuous throughout, becomes separated into lobules, and the network of cell-cylinders tends with multiplication of their cells to become fused into a continuous mass within each lobule, the bile canaliculi forming by numerous lateral junctions and anastomoses a close network of intercellular passages within the lobule.

The gall bladder and cystic duct are formed by a diverticulum from the common bile duct, which appears in the second month.

In the elasmobranch fishes, and in amphibia, there is only a single hepatic diverticulum. The anastomosing cell-cylinders which sprout from this are not solid, but hollow, with a narrow lumen, and the liver has from the first and retains permanently the character of a compound gland formed of anastomosing tubules. In reptiles the cylinders also have from the first a narrow lumen. In birds and mammals the cylinders are solid, as in man.

As the liver grows, it projects on either side into the pleuroperitoneal cavity. The mesoblast which unites it to the anterior wall of the abdomen, becomes thinned out to form

the suspensory ligament. That which unites it to the ventral wall of the stomach and duodenum also becomes thinned out; it forms the small or gastro-hepatic omentum. The liver is at first an exactly symmetrical organ, the right and left lobes being equal in size and symmetrically placed. After the fourth month the right lobe begins relatively to increase in size, and at birth the proportion of this to the left lobe is as 16 to 1. The liver also at first grows very rapidly, so that by the second month it nearly fills the abdomen, and causes a well marked prominence on the ventral aspect of the embryo. At this time it is calculated to constitute nearly one half the weight of the body. The proportion, however, gradually decreases, until at term the relative weight of the liver to the whole body is as 1 to 18. The further changes which the blood-vessels which pass to the liver undergo will be considered with the development of the venous system.

The pancreas is developed as a hollow hypoblastic diverticulum from the dorsal wall of the duodenum opposite the hepatic diverticula, and somewhat later than these (fig. 125, B, C, D, p). This hollow process grows into the mesogastrium or gastroduodenal mesentery, which at this time is well developed, and ramifies within this, producing by its off-shoots the ducts and alveoli as with other compound acinous glands. As the duodenal loop becomes formed, and this and the pyloric end of the stomach are turned over towards the right side, the pancreas loses its median symmetrical position, and with the mesentery which encloses it now lies across the back of the abdomen. This is the condition in which the gland is found in most mammals. But in man, with the fusion of the mesogastrium (great omentum) to the transverse mesocolon, the posterior layer of the mesenteric fold which encloses the pancreas becomes absorbed (Toldt), and the gland becomes fixed across the back of the abdomen, and is now apparently altogether behind the peritoneum (see fig. 12, 129).

RECENT LITERATURE.

Bemmelen, J. F. van, Entwikkeling en metamorphose der kieuw of viceral-spalten en der aortabogen bij embryonen van Tropidonotus natrix en Lacerta muralis, Kin. Akad. v. Wet. Amsterd. Afd. Natuusk., 1885; Die Visceraltaschen u. Aortenbogen bei Reptilien u. Vögeln, Zool. Anzeiger, 1886; Die Halsgegend der Reptilien, Zool. Anz., 1887.

Bonnet, R., Ueber die Entwicklung der Allantois und die Bildung des Afters bei den Wiederkäuern und über die Bedeutung der Primitivrinne und des Primitivstreifs bei den Embryonen der Säugethiere, Anat. Anzeiger, 1888.

Born, G., Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten, Archiv f. mikr. Anat., Bd. xxii., 1883.

Cadiat, Du développement des fentes et arcs branchiaux chez l'embryon, Journal de l'anat., &c.,

1883.

Chievitz, J. C., Beiträge zur Entwicklungsgeschichte der Speicheldrüsen, Arch. f. Anat. u. Physiol., Anat. Abtheil., 1885.

Demon, F., Développement de la portion sousdiaphragmatique du tube digestif. Lille, 1884. Dohrn, A., Die Thyroidea bei Petromyzon, Amphioxus u. Tunicaten, Mitth. aus der zool. Station z. Neapel, 1886.

Fischelis, Ph., Beiträge zur Kenntniss der Entwicklungsgeschichte der Gl. thyreoidea u. Gl. thymus, Arch. f. mikr. Anat., Bd. xxv., 1885.

His, W., Ueber den Sinus præcervicalis und die Thymusanlage, Archiv f. Anat. u. Physiol., Anat. Abth., 1886; Zur Bildungsgeschichte der Lungen beim menschlichen Embryo, Archiv f. Anat. und Physiol., Anat. Abtheilung, 1887; Schlundspalten u. Thymusanlage (Brief an F. Mall), Arch. f. Anat. u. Physiol., Anat. Abth., 1889.

Kastschenko, N., Das Schicksal der embryonalen Schlundspalten bei Säugethieren, Archiv f. mikrosk. Anat., Bd. xxx., 1887; Das Schlundspaltengebiet des Hühnchens, Arch. f. Anat. u. Phys. Anat. Abth., 1887.

Liessner, E., Ein Beitrag zur Kenntniss der Kiemenspalten und ihrer Anlagen bei amnioten Wirbelthieren, Morpholog. Jahrbuch, Bd. xiii., 1888.

Mall, F. P., Entwicklung der Branchialbogen und Spalten des Hühnchens, Arch. f. Anat. u. Physiol., Anat. Abth., 1887; The branchial clefts of the dog, with special reference to the origin of the thymus gland, Studies from the Biol. Laboratory of John Hopkins University, iv., 1888.

Meuron, P. de, Recherches sur le développement du thymus et de la glande thyroïde, Recueil zool. Suisse, iii., 1886; Sur le développement de l'œsophage, Compt. rend., 1886.

Minot, Ch. S., Evolution of the Lungs, Proceed. of the Zoolog. Society of London, 1886.

Ostroumoff, A., Ueber den Blastoporus u. d. Schwanzdarm bei Eidechsen u. Selachiern, Zool. Anzeiger, 1889.

Philip, R. W., Beiträge zur Lehre über die Entwicklung der Trachea, Mitth. aus d. embryol. Inst. d. Univers. Wien, Bd. ii., 1883.

VOL. I.

I

Piersol, G. A., Ueber die Entwicklung der embryonalen Schlundspalten und ihre Derivate bei Säugethieren, Zeitschr. f. wiss. Zool., Bd. xlvii., 1888.

Rabl, C., Zur Bildungsgeschichte des Halses, Prager medic. Wochenschr., 1886 u. 1887. Retterer, E., Du développement de la région anale des mammifères, C. r. de la société de biologie,

1890.

Robinson, A., Observations on the earlier stages in the development of the lungs of rats and mice, Journal of Anatomy and Physiology, 1889.

Schwink, T., Ueber den Zwischenkiefer und seine Nachbarorgane bei Säugethieren, 1888.

Stieda, Untersuchungen ueber die Entwickl. der Glandula thymus, Glandula thyoidea und Glandula carotica. Leipzig, 1881.

Swartz, D., Untersuchungen des Schwanzendes bei den Embryonen der Wirbelthiere, Zeitsch. f. wiss. Zool. xlviii., 1889.

Toldt, C., Bau u. Wachsthumsveränderungen der Gekröse des menschlichen Darmkanales, Wiener Denkschriften, 1879; Die Darmgekröse u. Netze im gesetzmässigen u. im gesetzwidrigen Zustand. Ibid., 1889.

Uskow, N., Bemerkungen zur Entwicklungsgeschichte der Leber und der Lungen, Archiv f. mikrosk. Anatomie, Bd. xxii., 1883.

Wölfler, A., Ueber die Entwickl. u. den Bau der Schildrüse. Berlin, 1880.

DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS. 115

DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS.

The urinary and generative organs originate in connection with the intermediate cell-mass, a portion of mesoblast which is seen in sections of the early embryo lying

Fig. 136.-PART OF A TRANSVERSE SECTION OF A CHICK EMBRYO OF 2 DAYS, 6 HOURS. (Kölliker.) 250

uw, protovertebra; mp, lateral mesoblast; dfp, splanchnopleuric mesoblast; hp, somatopleuric mesoblast; p, pleuro-peritoneal cleft (cœlom); wg, Wolffian duct; wk, part of intermediate cell-mass from which Wolffian body will become developed.

between the paraxial mesoblast and the pleuro-peritoneal cleft, and abutting against the external epiblast (fig. 39, p. 37).

me

де

Some of the cells of this intermediate cell-mass become differentiated into a longitudinally running cord, which subsequently acquires a lumen, and is then known as the Wolffian duct (from its discoverer, Caspar Friedrich Wolff) (fig.136, wg). Posteriorly the duct opens into the cloaca. The anterior part of the duct becomes connected with invaginations of the peritoneal epithelium, between which vascular glomeruli project freely into the peritoneal cavity (fig. 137). These glomeruli constitute the head kidney, fore-kidney or pronephros. Along its inner side, somewhat further back

1 Hertwig. According to Balfour and Sedgwick, these glomeruli form the anterior part of the Wolffian body, and the head kidney is represented by the Müllerian invaginations referred to later on (see p. 122 and fig. 145).

116 DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS.

wards, a series of transversely coursing tubes becomes developed in the intermediate cell-mass. These tubes are connected for a time with other involutions of the peritoneal epithelium (fig. 141), but subsequently lose their connection with that

Fig. 138. DIAGRAMS OF THE ARRANGEMENT OF THE URINARY AND GENITAL ORGANS IN ELASMOBRANCHS.

(Palfour.)

A.-DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN ELASMOBRANCH EMBRYO.

pd, segmental duct; opening at o, into the body cavity and at its other extremity into the cloaca ; r, line of separation between the Wolffian duct above and the Müllerian duct below; st, segmental tubes, opening at one end into the body cavity and at the other into the segmental duct.

B.-DIAGRAM OF THE ARRANGEMENT OF THE URINO-GENITAL ORGANS IN AN ADULT FEMALE

ELASMOBRANCH.

m.d, Müllerian duct; w.d, Wolffian duct; s.t, segmental tubes; five of them are represented with openings into the body cavity, and five posteriorly correspond to the metanephros; or, the ovary; d,

ureter.

C.-DIAGRAM OF THE ARRANGEMENT OF THE URINO-GENITAL ORGANS IN AN ADULT MALE ELASMOBRANCH.

m.d, rudiment of Müllerian duct; w.d, Wolffian duct, serving at vd as vas deferens; s.t, segmental tubes, two represented with openings into the body cavity; d, ureter; t, testis; nt, canal at the base of the testis; V.E, vas efferentia; le, longitudinal canal of the Wolffian body.

epithelium, and acquiring glomeruli at one part, at another part open into the Wolffian duct. They form the mid-kidney, Wolffian body or mesonephros, which