THE ALLANTOIS. 105 



The size of the allantoic cavity in unguiculates varies considerably. In man 

 it is minimal, forming only a long and very narrow tube (compare page 138). In 

 rodents it expands somewhat, but it never becomes free in the sense that it is 

 separated from the body-stalk, although it may acquire a partial independence. 

 In this case it may also become more or less vascular by the development of 

 branches from the umbilical arteries and veins around the allantois. 



In those animals in which the allantois is free, the umbilical arteries and 

 veins have all their branches in the allantois, there being no body-stalk. The 

 embryo is without connection with the chorion, and, therefore, these vessels in 

 their ramifications are restricted to the allantois. 



Relations of the Allantois to the Chorion in Ungulates. — Since the true chorion 

 is the outermost of the foetal envelopes, it alone can come in contact with the walls 

 of the uterus. All placental developments, therefore, necessarily depend upon 

 the chorion. Now, in ungulates, where the chorion is without blood-vessels, 

 there is no physiological apparatus to transfer any nutritive material which may 

 be taken up by the chorion from the uterus to the embryo until a second union 

 takes place between the vascularized allantois and the chorion. The inner sur- 

 face of the chorion and the outer surface of the allantois are both mesodermic. 

 The two mesodermic layers come into contact with one another and unite loosely. 

 The vessels of the allantoic mesoderm are thus brought into physiological union 

 with the chorion, but, being allantoic vessels, they are, of course, morphologically -^ 

 different from the chorionic vessels of unguiculate mammals. These considera- 

 tions demonstrate that the ungui^uJate placenta is allantoic rather than 

 chorionic, and is, morphologically speaking, essentially different from the true 

 chorionic placenta, which can be developed only in those animals and embryos 

 which have a permanent body-stalk. 



The simple relations of the chorion in the ungulata to the uterine wall is 

 illustrated by the accompanying figure 49, which shows a portion of the chorion 

 of a pig embryo of 1 5 mm., together with the surface of the uterus to which it was 

 fitted. The two membranes were accidentally separated in the preparation. 

 The chorion consists of a layer of cylinder epithelial cells, Ec, each of which can 

 be distinctly made out, and of a layer of mesoderm, Mes, containing only few 

 cells and blood-vessels, two of which, Ve, are shown in the section; the meso- 

 dermic cells are a little more crowded near the epithelium. Each ectodermal 

 cell is distinctly marked off from its neighbors by a line. The protoplasm stains 

 somewhat, the nuclei are slightly oval and granular, and are situated near the 

 middle of the cells. The top of each cell is concave. The uterine epithelium, 

 L't. Ep, resembles in the general form of its cells and in the character of its proto- 

 plasm the chorionic ectoderm, but differs from it in that each cell has a convex 



