208 PLACENTA OF THE UNGULATA. 



derived from the primitive type (p. 199) by an extension of the allantoic 

 portion of the chorion ; and the exclusion of the yolk-sack from any par- 

 ticipation in forming the chorion. 



The possession in common of a diffused type of placenta is by no 

 means to be regarded as a necessai-y proof of affinity between two groups, 

 and there are often, even amongst animals possessing a diftused form of 

 placenta, considerable differences in the general arrangement of the em- 

 bryonic membranes. 



Ung'ulata. Although the Ungulata include forms with both coty- 

 ledonaty and diffused placentse, the general arrangement of the embryonic 

 membranes is so similar throughout the group, that it will be convenient to 

 commence with a description of them, which will fairly apply both to the 

 Ruminantia and to the other forms. 



The blastodermic vesicle during the early stages of development lies 

 freely in the uterus; and no non-vascular villi, similar to those of the 

 Dog or the Rabbit, are formed before the appearance of the allantois. 

 The blastodermic vesicle has at first the usual spherical form, but it grows 

 out at an early period, and with prodigious rapidity, into two immensely 

 long horns ; which in cases where there is only one embryo are eventually 

 prolonged for the whole length of the two horns of the uterus. The 

 embryonic area is formed in the usual way, and its long axis is placed at 



Fig. 153. Embkyo and fcetal membbanes of a young embryo Roe-deer. (After 



Bischoff.) 

 yk. yolk-sack ; all. allantois just sprouting as a bilobed sack. 



right angles to that of the vesicle. On the formation of an amnion there 

 is formed the usual subzonal membrane, which soon becomes sej^arated by 

 a considerable space from the yolk-sack (fig. 153). The yolk-sack is, how- 

 ever, continued into two elongated processes (y/c), which pass to the two 

 extremities of the subzonal membrane. It is supplied with the normal 

 blood-vessels. As soon as the allantois appears (fig. 153 all), it grows out 

 into a right and a left process, which rapidly fill the whole free space within 

 the subzonal membrane and in many cases, e.q. the Pig (Von Baer), break 

 through the ends of the membrane, from which they project as the diver- 



