118 



LECTUEES ON BIOLOGY 



the archetypal fish Branchiostoma, the process is in every case 

 exactly similar. 



The one-layer germ forms the two-layer germ, the gastrula, 

 by folding one wall of the cell-vesicle against the other. It must 

 be understood that we are considering here only the most fre- 

 quent form of development, as observed in the most distantly 

 related species. In numerous other cases the process of gastru- 

 lation shows considerable differences, and very frequently the 

 gastrula stage can only be demonstrated by accurate comparative 

 examination. It seems obvious that this should be so, for an 

 egg with -a large accumulation of yolk cannot divide in the same 

 manner as an egg which, having no yolk, offers less resistance to 

 the process of division. 



FIG. 32. EMBRYOS OF VERTEBRATES. 



(1) Budgerigar (Melopsittacus undulatus) (after Parker) ; (2) Cercopithecus 

 cynomolgus (after Lalenka) ; (3) Man (after His). 



The gastrula stage is of particular importance to the Evolution 

 Theory, for not only do we meet with it in the embryonic stages 

 of the most widely differentiated stems, but there even exists 

 a fully developed animal which permanently remains at this 

 stage, the Protohydra, belonging to the Coalenterates. Its whole 

 organization consists of a stomach cavity surrounded by a double 

 cell-mantle, having an orifice at the thinner end which serves 

 both as mouth and anus. 



Similar resemblances are observed in the higher stages of the 

 embryonic development, thus impelling us to the hypothesis 

 of a common ancestor. Even superficial comparison of the 

 embryos of different classes of the vertebrates prove this striking 



