FERTILIZATION 177 



one phase of a general physical and chemical reorganization 

 of the whole substance of the egg, following the entrance of the 

 spermatozoon. The egg may exhibit more or less amoeboid 

 movement, or waves of contraction may pass over it. A 

 frequent result is seen in the rapid streaming of differentiated 

 cytoplasmic substances into certain regions, where these specific 

 substances collect. Thus in many yolk-filled eggs like those of 

 the Teleosts, the protoplasm, which before the entrance of the 

 sperm is quite uniformly distributed over the surface of the 

 egg as a very thin layer, now collects at the animal pole into 

 a thick and fairly circumscribed disc called the germ disc (Fig. 

 48). The Ascidian egg, as described by Conklin, offers one of 

 the most marked examples of this rapid transformation and 

 redistribution of the substances of the egg cytoplasm (Figs. 

 91, 92). In the secondary oocyte of Cynthia (Styela) the 

 greater part of the cell is composed of a gray "endoplasm"; 

 superficially there is a thin but complete layer of yellowish 

 "mesoplasm"; while the large nucleus or germinal vesicle 

 contains a clear " ectoplasm." During maturation, which here 

 precedes sperm entrance, the ectoplasm collects at the upper 

 pole of the oocyte. Immediately upon entrance of the sperm 

 the yellow mesoplasm streams from all directions toward the 

 lower pole; this is followed by the clear ectoplasm which forms 

 a stratum just above the mesoplasm, and leaves the upper 

 half or more of the egg cytoplasm composed entirely of the gray 

 endoplasm. Then this radial or rotatory symmetry gives 

 place to a bilateral symmetry, for the mesoplasm and ecto- 

 plasm move up on one side (the posterior) of the egg, appearing 

 on the surface in the form of a crescent just below the equator. 

 Meanwhile the yellow mesoplasm and gray endoplasm have 

 each become differentiated into two distinct substances, so 

 that altogether five forms of protoplasm are distinguishable 

 in the cytoplasm of the zygote (Fig. 92). 



Very few eggs exhibit such marked differentiation as this, 

 but the corresponding phenomena are of widespread occurrence, 

 and it is quite likely that they have frequently been overlooked 

 because the various substances are not often marked by 



