50 Jenkinson, Germinal Layers of Vertebrates. 



MAMMALIA, 



The egg of the oviparous Monotremata is megalecithal 

 and meroblastic. At the close of segmentation there are 

 two layers in the blastoderm — an upper layer and a 

 paraderm ; beyond this, nothing of importance is known 

 of the germinal layers. 



The Qgg of the Placental Mammals, on the other hand, 

 is so small as to be almost alecithal ; but there is abundant 

 justification for the view that these Mammals are descended 

 from forms with large-yolked eggs, a justification which 

 is largely based on the mode of formation of the germinal 

 layers, which very closely resembles that observed in the 

 Reptiles and Birds, especially in the former. 



The germinal layers, however, only begin to be formed 

 after another — quite peculiar — process has taken place ; 

 andasthis process has — in my opinion, quite erroneously — 

 been confounded more than once with ' gastrulation,' or 

 a part of gastrulation, I may perhaps be allowed to state, 

 or rather re-state, the view which I have been led to adopt 

 on this matter. 



The fully segmented ovum consists of an outer layer 

 of cells surrounding an inner mass. The outer layer — 

 known as the 'trophoblast,' from the part it subsequently 

 plays in the formation of the placenta — is the homologue 

 of the false amnion, strictly speaking of the ectoderm of 

 the false amnion, of the Sauropsida ; the inner mass 

 contains within itself all the material necessary for the 

 production of the true amnion, the embryo, and the yolk- 

 sac. A cavity next appears separating the inner mass 

 from the outer layer at all points except one, the embry- 

 onic pole, and the ovum is now termed a ' blastocyst ' ; 

 at the same time the inner mass becomes differentiated 

 into an ' embryonic knob ' attached to the trophoblast. 



