STUDY OF SECTIONS OF EMBRYO OF 20 MM. 311 



Mes. On the ventral side of the cord is the cavity of the allantois, All, lined by 

 a thin layer of entoderm, and with no marked condensation of the mesenchyma 

 around it. A little lower is the large umbilical vein, U.V.S, which appears as a 

 prolongation of the left umbilical of the body proper, but the part within the cord 

 is probably the product, of the fusion of the two original veins. A little higher are 

 the two umbilical arteries, Art, which lie symmetrically as regards the allantois. 

 All three vessels are strengthened by walls of condensed mesenchyma, which is 

 much more prominent around the arteries than around the vein. The center of 

 the cord is occupied by a large irregular space, Cce, a prolongation of the body 

 cavity. In this umbilical ccelom are lodged the loop of the intestine and the cord 

 containing the vitelline vein. The intestine is cut twice, the section on the left 

 passing through the ileum, II, and on the right through the jejunum, which is 

 much larger than the ileum, having both a larger entodermal portion and a thicker 

 mesodermal part. The two segments of the intestine are joined together, and in 

 the part between them are two blood-vessels, one, the inferior, is the vitelline 

 artery, A.vi, which extends beyond the intestinal loop, to ramify upon the yolk- 

 sac; the other vessel is the superior mesenteric vein, which does not extend be- 

 yond the intestine. The mesenchyma of the intestines and of the bit of mesentery 

 between them consists of very crowded cells, so that the tissue appears darkly 

 stained. Above the loop lies the cord in which is situated the vitelline vein, V.vi. 

 The vein is centrally placed; the cord forms a thick wall of very loose mesenchyma 

 . covered by a thin mesothelial layer. The cord is a very characteristic embryonic 

 structure; it arises from the mesentery of the duodenum and extends through the 

 umbilical opening to the yolk-sac. The vein which it contains is thought to 

 arise by the fusion of the two original vitelline or omphalo-mesaraic veins. Its 

 union with the superior mesenteric vein to form the portal vein is shown in figure 

 100. All the surfaces of the ccelom are covered by a distinct mesothelium. The 

 main tissue of the umbilical cord is a typical loose mesenchyma, Mes. The ecto- 

 derm, EC, is the direct prolongation of the embryonic epidermis, and consists for 

 the most part of a single layer of cells, although the formation of a second outer 

 layer seems to be beginning. 



Pig Embryo of 20 mm. Study of Sections. 



Nine sections of this stage are figured. In the practical laboratory work em- 

 bryos a little larger or smaller may serve equally well to illustrate the develop- 

 mental conditions of this stage. 



Transverse Section through the Snout (Fig. 212). The parts shown are the same 

 as in figure 219, to the description of which reference is made. The present figure 

 212 is added to illustrate the development of the palate shelf, Pal. The palate 

 shelf is a large protuberance on the inner side of the maxillary process. Its inner 

 edge abuts against the tongue, Ton, its upper edge underlies the maxillo-turbinal 

 fold, max.tb, and its lower edge forms part of the roof of the oral cavity. Or. 



