EMBRYO OF 9 MM. STUDIED IN SECTIONS. 257 



channel through the liver rapidly enlarges and becomes recognizable as the vena 

 cava inferior. This important venous trunk is a combined vessel, comprising, first, 

 a part of the sinus venosus of the heart; second, the vena hepatica communis; third, 

 a large channel developed from the sinusoids of the liver; fourth, the upper part of 

 the right sub-cardinal vein; and, fifth, the lower part of the right cardinal. The 

 vena cava inferior has already been developed in the pig embryo of 9 mm. Be- 

 tween the two Wolffian bodies hangs down the large intestine, Reel, suspended by 

 its mesentery in the median line. The entodermal portion is a very small circle 

 of epithelium with an extremely minute lumen, which in the section is scarcely 

 larger than a single nucleus. The mesentery and intestine are covered by a well- 

 defined mesothelium and have a considerable amount of mesenchyma, in which 

 there is no distinct histological differentiation beyond the presence of a number 

 of small blood-vessels. At this stage the large intestine runs nearly in the median 

 plane to the pelvic end of the body. In the opposite direction, toward the head, 

 it bends to the left of the embryo, making a loop which passes over into the end of 

 the 1 ileum. The ileum forms the continuation of the loop and extends into the 

 ccelom at the bas'e of the umbilical cord. There it bends back and returns toward 

 the dorsal side of the embryo to pass over into the duodenum and join the stomach. 

 Owing to the fact that the small intestine extends into the extra-embryonic ccelom 

 of the umbilical cord, there makes a loop, and returns to the embryonic region, 

 we get typically a double section of the intestine as shown in the figure, one of 

 each limb of the loop. The entoderm, In, in these loops forms ^ small ring, which, 

 however, is much larger than the entodermal ring of the la. ui.c at this 



stage. Each loop contains a large amount of mesenchyma, mes. th< 

 are somewhat crowded, so that the tissue appears dark in the stained* section, 

 boundary between the body of the embryo and the tissue of i! 

 marked by the position of the two umbilical veins, that of the lett : 

 being very much larger than that of the right side, V.U.D. By following do., 

 the somatopleure, Som, of the embryo, it will be seen that these veins are lodged 

 therein, and that the continuation of the somatopleure beyond these veins forms 

 the substance of the umbilical cord. The limb-bud, P.L, is a large mass of 

 rather dense mesenchyma, entirely without muscles or nerves and covered by ecto- 

 derm. At the edge of the limb-bud the ectoderm shows a special thickening, F. 

 The theory has been advanced that this thickening is homologous with the ecto- 

 dermal fold -which produces the fin of fishes, or at least that portion of the fin in 

 which the fin-rays are developed. 



Frontal Section through the Second and Third Gill-Clefts 

 preparation the section hits the posterior wall, Ot, of t: 

 anterior to the origin of the glosso-pharyngeal nerve. The appe; 

 section ol the hind-br?in is characteristic for this region of young n 

 deck-plate has grown gradually in size and forms a wide n 

 the ependyrhal roc; !i e fourth ventricle. Owing to this 





