THE NUTRITIVE YELK. 221 



gastrula {Disco-gastrula , Fig. 48). We may therefore infer 

 that the latter is directly, or through the intermediate stage 

 of the Hood-gastrula, descended from the original Eell- 

 gastrula. It arose phylogenetically owing to the fact that 

 a store of nutritive matter collected at one pole of the egg, 

 and thus formed a nutritive yelk distinct from the forma- 

 tive yelk. Yet, notwitlistanding this, the Gastrula in this, 

 as in the former cases, was originated by an inversion or 

 invagination of the Blastula. We may, therefore, also refer 

 this kenogenetic form of discoidal cleavage {segmentatio 

 discoidalis) to the original and palingenetic form. 



Although it is thus tolerably easy and safe to trace back 

 the descent of the small egg of this oceanic Osseous Fish, yet, 

 on the other hand, it seems hard to do this with certainty 

 in the case of larger eggs, such as occur in the case of most 

 other Fishes, and in the case of all Reptiles and Birds. In 

 the first place, the nutritive yelk of these is quite dispro- 

 portionately large ; so large, indeed, that it almost causes 

 the formative yelk to disappear. And, in the second place, 

 the nutritive yelk contains a number of variously formed 

 constituent parts, which are known as the yelk-granules, 

 yelk-globules, yelk-vesicles, and so on. These definite yelk- 

 elements have often even been explained as true cells, 

 and it has been quite wrongly assumed that a portion 

 of the body of the embryo is found in them.'^^ Tliis 

 is by no means the case. The nutritive yelk, what- 

 ever its size, always remains a lifeless store of nutritive 

 matter, which, in the process of germination, is taken into 

 the intestine during its development, and is consumed by 

 the embryo. The latter develops solely from the living 

 formative yelk, from the parent-cell. This is equally true 



