1882.] Blastodermic Layers in the Rat and Mouse, 435 



Fig. 4. 



Section of ovuiii from rat, 9 days 17 hours. 



solid mass of cells, which forms a considerable part of the structure 

 of the foetal placenta, and which comes into relation with the allantois 

 in a manner to be presently described. 



The allantois consisting of a solid mass of mesoblastic cells, in wbich 

 there is no hypoblastic dilatation, grows outwards from the posterior 

 end of the embryo into the above-mentioned inter- amniotic space. 

 (Fig. 4, all.) A little over the eleventh day it reaches the mass of cells 

 formed from the false amnion, while its vessels formed in the same 

 manner as in other animals, pass through the mass of cells and pene- 

 trate the maternal part of the placenta, accompanied by extensions of 

 the cellular structure derived from the placental pole of the blasto- 

 dermic vesicle. The alimentary canal begins to be separated from the 

 general hypoblast in the form of a groove which up to the twelfth 

 day opens freely into the space between the hypoblast, and the wall 

 of the blastodermic vesicle. The peripheral portion of hypoblast 

 outside the alimentary groove, which is equivalent to the yolk sac of 

 other animals, and becomes vascular in a similar manner by the meso- 

 blastic lining on its inner surface, becomes thrown into villous pro- 



