THE ALL AN TO IS. 113 



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

 it is minimal, forming only a long and very narrow tube (compare Fig. 66, All). 

 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 rami- 

 fications are restricted to the allantois. 



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

 the outermost of the fetal 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 circulatory 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 surface 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 differ- 

 ent from the chorionic vessels of unguiculate mammals. These considerations 

 demonstrate that the ungulate 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 perma- 

 nent body-stalk. 



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

 illustrated by the accompanying figure 65, which shows a portion of the chorion 

 of a pig embryo of 15 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 mesodermic 

 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 some- 

 what; 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, Ut.Ep, 

 resembles in the general form of its cells and in the character of its protoplasm 

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

 and, further, in that the nuclei of the cells are situated near the top of the layer. 

 When the relations of the two epithelia have not been disturbed, it is readily 

 observed that the concavity of each chorionic ectodermal cell receives the convex 



