ALLANTOIS AND PLACENTA. 1371 



Up to the seventh clay the blastodermic vesicle lies quite 

 freely in the uterus (Fig. 168), but towards the end of the 

 seventh Jay it begins to acquire adhesions to the uterine wall. 

 These are effected partly by the small epiblastic villi of the lower 

 pole of the vesicle (Fig. 146, EK), but principally by the epiblast 

 cells of the vascular area : these latter proliferating freely over a 

 horse-shoe-shaped patch (Fig. 145, E'), which surrounds the sides 

 and hinder end of the embryonic region ; and growing out into 

 irregular tags and processes, which adhere firmly to the wall of 

 the uterus. By the ninth day (Figs. 145, 146, and 169), this 

 adhesion has become so firm, that, if the blastodermic vesicle is 

 pulled away from the uterus, the thickened epithelium over this 

 horse-shoe area is torn from the vesicle and remains attached to 

 the uterine wall. 



By the tenth day (Fig. 147), the allantois has come into 

 extensive contact with the wall of the blastodermic vesicle 

 beneath this area of attachment ; and the blood-vessels of the 

 allantois are thus brought very close to the uterine vessels of 

 the mother. By a further series of changes, which will be 

 fully described later on in this chapter, the surface of contact 

 between the maternal and foetal blood-vessels is greatly in- 

 creased, and the highly complicated structure of the placenta is 

 gradually elaborated (cf. Fig. 170). 



DEVELOPMENT OF THE NERVOUS SYSTEM. 

 1 . General Account. 



In the rabbit, as in the chick and other Vertebrates, the 

 nervous system is established very early. The neural groove 

 (Fig. 144, NG) appears, about halfway through the eighth day, 

 as a shallow longitudinal depression, in front of the primitive 

 streak, and bordered laterally by the neural folds. 



The neural groove rapidly increases, both in length and in 

 depth. By the end of the ninth day (Fig. 145) the lips of the 

 groove have met and fused along the greater part of their length, 

 though still remaining separate at both the anterior and pos- 

 terior ends. The distinction between the wider anterior part, or 

 brain, and the narrower posterior portion, or spinal cord, is very 

 evident ; and the vesicles of the fore-brain, mid-brain, BM, and 

 hind-brain are already well established. 



