310 



STUDY OF PIG EMBRYOS. 



is suspended the large intestine. It has a small canal formed by entoderm and 

 very thick mesodermic walls. 



Attached to the ventral side of the body- wall of the embryo is the allantois, 

 All, the cavity of which is quite large, somewhat irregular in shape, and lined 

 by a cuboidal epithelium, a portion of the entoderm. By following through the 

 sections it can be seen that the allantois and large intestine join at the cloaca. 

 The entodermal allantois is surrounded by mesenchyma, which is very much 

 looser in texture than that of the intestine proper. On either side of the allantois 

 is a projecting lobe of tissue in which the umbilical artery, A.um, is lodged. The 



A.m. 



V.vi. 



Cce. 



Art. 



U.V.S. All. 



FIG. 211. PIG, 17.0 MM. FRONTAL SERIES 39, SECTION 64. 



A II, Allantois. Art, Umbilical artery. A .vi, Vitelline artery. Cce, Ccelom. EC, Ectoderm. //, Ileum. mes, 

 Mesenchyma. U.V.S, Left umbilical vein. V.vi, Vitelline vein. X 35 diams. 



two arteries pass upward to the umbilicus, then outward to the placenta. Down- 

 ward they continue to the level of the cloaca, there pass to the dorsal side of 

 the embryo, and unite with the end of the median dorsal aorta. 



Frontal Section of the Umbilical Cord (Fig. 211). We get in frontaj series 

 of the embryo sections of the umbilical cord which are more or less nearly trans- 

 verse. The major part of the area of such sections is occupied by mesenchyma, 





