Manchester Memoirs, Vol. I. ( 1 906), No. 3. 



51 



and a layer of flattened cells, the paraderm, which begins 

 to grow round the blastocystic cavity. Segmentation 

 therefore is followed in the Placentalia by the separation 

 of the elements of the trophoblast from those destined to 

 give rise to the embryo and the remainder of its foetal 

 membranes, and this ' precocious segregation ' seems to 

 have occurred phylogenetically during the gradual loss of 

 yolk which the egg of these Mammals has undergone. 



Fig. 31. 

 Formation of the germ layers in Mammals. 



A. Vespertilio (after Van Beneden). Sagittal section showing the 

 archenteron opening to the exterior behind the dorsal lip, and communicating 

 with the subgerminal cavity {s.g.c.) below. A small portion of the floor 

 remains. 



B. Transverse section of the embryonic area of the mouse. (Original). 

 The roof of the archenteron (which will become the notochord) is inserted 

 by its edges into the paraderm ; from its sides spring two lateral sheets of 

 mesoderm. 



From a part of the embryonic knob the true amnion 

 is eventually formed, in a manner which varies in different 

 types. The rest of this knob becomes the upper layer of 

 the embryonic area, and is the equivalent of the 

 embryonic shield of the blastoderm in the Sauropsida. 



The development of the germinal layers now proceeds. 

 As in the Sauropsida, the necessary material is brought 



