230 THE EVOLUTION OF MjVN. 



(Blastoderma, Fig. 28, b). This germ-membrane is a simple, 

 completely closed vesicle, the space within being wholly 

 filled with nutritive yelk. The chemical quality of the 

 contents of this true germ-vesicle, or Blastula (Fig. 28) 

 alone distinguishes it from the Blastula of the primordial 

 cleavage-process (Plate II. Fig. 4). The latter contains 

 water, or jelly as transparent as water ; the former con- 

 tains a dense mixture of albuminous and fatty substances, 

 in which there is much nutritive matter. As this extensive 

 nutritive yelk occupies the centre of the egg from the very 

 beginning of the cleavage, there is naturally no difference in 

 this case between the mulbeiTy-gcrm and the vesicular 

 germ. 



When the germ-vesicle (Fig. 28) is quite complete, the 

 important process of inversion {invaginatio), which produces 

 the Gastrula, follows (Fig. 29). A circvdar, groove-like 

 deepening first arises at a point on the surface, and this 

 enlarges into a cavity, the primitive intestinal cavity of 

 the Gastrula (Fig. 29, d) ; the point at which the inversion 

 takes place forming the primitive mouth of this cavity (o). 

 The inverted portion of the germ-membrane, the cells of 

 which enlarge and assume a slender cylindrical form, consti- 

 tutes the intestinal layer and surrounds the cavity of the 

 primitive intestine. The superficial, uninverted portion of 

 the germ-membrane forms the skin-layer; the cells of this, 

 owing to continual self-division, become smaller and more 

 flattened. The sj^ace between the skin-layer and the intes- 

 tinal layer (the remnant of the cleavage-cavity) continues 

 full of nutritive yelk, which is now gradually consumed. 

 This is the only essential point in which the Bladder- 

 gastrula {Peri-gastrida, Fig. 29) differs from the original 



