8 E. E. JUST. 



Nor, again, is mere sperm penetration fertilization, since sperm 

 normally penetrate ova (Dinophilus, Saccocirrus, etc.) some time 

 before fertilization ensues. There are thus all possible types of 

 fertilization with respect to the maturation stage of the egg when 

 normally inseminated. No definition of fertilization is worth 

 while if based on one type of egg alone. 



In the second place, though the end result of fertilization is 

 cleavage, there are here, too, many differences among animal ova. 

 Thus the zygote nucleus may at first divide without cytoplasmic 

 division (Renitta) ; the germ nuclei may fuse or appose merely; 

 the cleavage spindle may be homodynamic, or heterodynamic ; the 

 sperm amphiaster may be homo- or heterodynamic, its second aster 

 arising before or after union with the egg nucleus ; and the cleav- 

 age centers may arise about the sperm nucleus or the egg nucleus 

 or in part about each. A definition of fertilization in terms of 

 the behavior of the germ nuclei or of the origin of the cleavage 

 centers is manifestly inadequate. 



If, for example, we consider the classic theory of Boveri that 

 fertilization is due to the introduction of centrosomes by the 

 spermatozoon, we realize its inadequacy at once, since it demands 

 that the middle-piece enter the egg. It is true that the whole 

 spermatozoon enters certain eggs whose maturation spindles are 

 without centrosomes or asters, thus apparently supplying a de- 

 ficiency. But in many other cases the middle-piece does not enter 

 the egg, and where it does as in echinid ova the identity of its 

 so-called centrosomes is wholly mistaken. To support the Boveri 

 hypothesis we must shift the position of the potent centrosomes to 

 fit those cases where the middle-piece does not enter the egg, or on 

 entering takes no part in aster formation. 



Because of failure to recall these simple facts purely morpho- 

 logical theories of fertilization fail. Indeed, many studies on fer- 

 tilization are but studies of cell division ; they deal with structures 

 and phenomena in cell division in no wise restricted to egg cells. 

 Nor yet have many physical or chemical theories been more fortu- 

 nate. These theories are based on the study of physiological 

 changes incident to cell division. But cell division is not fertili- 

 zation. 



An approach to the fertilization problem can be made only 



