THE NUTRITIVE YELK. 221 



astrula {Disco-gastrula, Fig. 43). We may therefore infer 

 hat the latter is directly, or through the intermediate stage 

 f the Hood-gastrula, descended from the original Bell- 

 astrula. It arose phylogenetically owing to the fact that 



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

 nd thus formed a nutritive yelk distinct from the forma- 

 Lve yelk. Yet, notwithstanding this, the Gastrula in this, 

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

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

 liis kenogenetic form of discoidal cleavage (scgmentatio 

 'iscoidalis) to the original and palingenetic form. 



Although it is thus tolerably easy and safe to trace back 

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

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

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

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

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

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

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

 he nutritive yelk contains a number of variously formed 

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



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

 lements have often even been explained as true cells, 

 ud it has been quite wrongly assumed that a portion 

 f the body of the embryo is found in them.'^^ TJiis 

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

 ver its size, always remains a lifeless store of nutritive 

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

 he intestine during its development, and is consumed by 

 he embryo. The latter develops solely from the living 

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



