2l6 THE EVOLUTION OF MAN. 



In all these animals, both the ripe egg-cell, and the parent- 

 cell, into which fertilization transforms this egg-cell, consist 

 of two quite distinct and separate parts, which are distin- 

 guished respectively as the formative yelk and the nutritive 

 yelk. The formative yelk {vitellus formativus, or miorjpho- 

 lecithus) is the nucleated egg-cell, capable of evolution, which 

 divides in the process of cleavage, and produces the nu- 

 merous cells which constitute the embryo. The nutritive 

 yelk (vitellus nutritivus, or tTOjjholecithus), on the other 

 hand, is a mere appendage of the true egg-cell, and contains 

 hoarded food-substance (albumen, fat, etc.); so that it forms 

 a sort of storehouse for the embryo in the course of its 

 evolution. The embryo absorbs a quantity of nutritive 

 matter from this storehouse, and finally entirely consumes it. 

 Indirectly, therefore, the nutritive yelk is of great import- 

 ance in germination. Directly, however, it takes no share 

 in the process, for it is not concerned in the cleavage, and 

 is not cellular. Sometimes the nutritive yeJk is smaller, 

 sometimes larger; generally many times larger than the 

 formative yelk ; for which reason, greater importance was 

 formerly attached to the nutritive than to the formative 

 yelk. All eggs which have this independent nutritive yelk, 

 and of which, therefore, only a portion undergoes cleavage, 

 are called meroblastic, the name given them by p,emak ; 

 their cleavage is incomplete or partial (Plate III.). *^ 



It is not easy correctly to apprehend this partial egg- 

 cleavage, and the peculiar form of Gastrula which results 

 from it; and it was only quite recently that comparative 

 research enabled me to remove tliis difficulty, and to retrace 

 this kenogenetic form of cleavage and gastrulation to the 

 original, palingenetic form. The sea eggs of one of the 



