THE YOLK-SAC AND AMNION. 369 



vascular, or original upper wall of the yolk-sac, comes into con- 

 tact with the wall of the uterus, the hypoblast of the yolk-sac 

 lying in contact with the uterine epithelium. 



About the eighth day, irregular epiblastic buds (Figs. 146, 

 117, KK) nrir-fe from the surface of the lower, or non-vascular, half 

 of the yolk-sac. These acquire close attachment to the mucous 

 membrane of the uterus, and aid in fixing the blastodermic 

 vesicle in position, while it is possible that they have also some 

 nutritive function. They begin to degenerate about the ninth 

 day, and by the fourteenth or fifteenth day have disappeared. 



j. The Amnion. 



The amnion of the rabbit, while agreeing in most respects 

 with that of the chick, differs from this in the prominent share 

 taken by the tail-fold, which, as was first pointed out by Van 

 Beneden and Julin, practically forms the whole amnionic 

 covering of the embryo. 



On the ninth day, as already mentioned, there is, immediately 

 in front of the head of the embryo, a patch of the blastoderm, 

 roughly circular in outline (Figs. 145, 146, AX'), into which the 

 rnesoblast does not yet extend, and which therefore consists of 

 epiblast and hypoblast alone. This patch is termed the pro- 

 amnion, and corresponds to the similarly named structure in the 

 chick. 



The rapid growth of the head of the embryo forwards, and 

 then downwards, depresses the pro-amnion so as to form a deep 

 pocket, projecting into the yolk-sac. This is a well-marked 

 feature on the tenth and eleventh days (Fig. 147, AN'), but 

 from the twelfth day onwards it becomes less obvious (Fig. 148), 

 owing to the general depression of the upper wall of the yolk- 

 sac which is then occurring. 



The pro-amnion, as a special part of the wall of the yolk- 

 sac, has only a temporary existence. The mesoblast gradually 

 invades it from the sides, spreading inwards between the 

 epiblast and hypoblast, and, on the three-layered condition 

 being definitely attained, the pro-amnion as such ceases to 

 exist. 



The amnion itself is formed almost entirely by the tail-fold, 

 aided to a slight extent by the side-folds. The tail-fold of 

 the amnion (Fig. 146, AN) is formed immediately behind the 



B B 



