LATER DEVELOPMENT OF CHICK AND RABBIT 439 



affect the relation of the membranes to the uterine wall, and so the 

 type of placenta formed, but it gives no indication of the enormous 

 variations that are to be met with in the details of the origin, forma- 

 tion and final constitution of this highly important structure. 

 These are matters of considerable complexity, and outside the scope 

 of this work, so that we can only consider the outlines of them in the 

 case of the rabbit. 



The uterine mucosa in the non-pregnant rabbit is arranged in a 

 series of longitudinal folds, of which two, situated one on each side 



Ex 



Ex 



YE 



FIG. 154. Diagram of section of fully formed blastodermic vesicle of 

 Lepus, adapted from Hertwig. 



A., Amnion ; Al., allantois ; Al.S., allantoic stalk ; E., embryo ; Ex., exoccel ; S.T., sinus 

 terminalis ; T., trophoblast ; V., villi ; V.L., vasuclar layer of yolk sac ; Y., yolk sac ; Y.E., yolk 

 sac entoderm ; Y.S., yolk sac stalk. 



of the line of attachment of the broad ligament, are larger and more 

 important than the remainder : they are termed the placental ridges. 

 They are separated by a groove, and generally when the blastocyst 

 adheres to the uterine wall it is in such a position that the embryo 

 is near and parallel to this groove. As we have seen previously, a 

 horse-shoe-shaped band of trophoderm is established at an early 

 date, and this lies in the region touching the ridges. The cells of this 

 delimited area multiply rapidly and fuse, so that they form a 

 syncytium, which is extremely active, and in which cell walls cannot 

 be made out. The uterus, and particularly the placental ridges, 



