THE HISTORY OF THE EGG FROM FERTILIZATION 



TO CLEAVAGE. 



The time between deposit and the appearance of the first cleavage 

 groove is usually not more than forty or fifty minutes. Thirty minutes 

 after fertilization the pronuclei are recognizable in the living egg as 

 two contiguous vesicles of about equal volume. In this short period, the 

 whole prelude to cleavage is performed, — the penetration of the sperma- 

 tozoon, the production of two polar globules, the development of the 

 pronuclei, and the formation of the blastodisc. It is a crowded and hur- 

 ried scene, in which varied and complex parts are simultaneously enacted 



The phenomena are usually roughly classified under three heads : 

 copulation, mattiration, and fecundation. It must be borne in mind, however. 

 that it is quite impossible in the present state of our knowledge to draw 

 sharp lines between these groups. We have at the very least four dif- 

 ferent substances, two nuclear and two cytoplasmic bodies ; and how 

 many active factors are represented in each is at present beyond our 

 calculation. The spermatic cytoplasm is usually a minim quantity, and 

 its history is so obscure that the more favorable objects of study have 

 failed to give any decisive information as to the part it plays, or even 

 to settle the question whether it has any function at all. The history 

 of this element in the fish egg has entirely escaped us, but we are far 

 from being convinced that it has no share in the phenomena displayed. 

 But the uncertainty in this regard is by no means the only source of 

 confusion. No less than three distinct elements are recognizable in every 

 nucleus; and that the cytoplasm contains active as well as passive elements 

 cannot be denied. Witli all these factors actinu; and interactiii"- simul- 

 taneously in the same field, we cannot wonder that scarcely two authors 

 can be named who would agree upon what should be called phenomena 

 of maturation and what phenomena of fecundation. In one egg certain 



