2V6 W. G. YanName — Embryology of Eu&tylochus. 



and slender end and the thicker and inactive part, probably the 

 middle piece, is visible, nor can any tail be distinguished. 



The spermatozoa are often present in the tissues in almost incredi- 

 ble numbers, and some difficult questions in regard to the details of 

 the process of fertilization present themselves. Although they can 

 penetrate most of the body-tissues without difficulty, there must be 

 others that are more or less impermeable to them, as for example 

 those of the male reproductive organs. Otherwise it would not be 

 possible for an individual to store up the fully developed spermato- 

 zoa, or to prevent fertilizing its own eggs. 



I have not seen the spermatophores of Planocera nehulosa and 

 therefore have not examined and measured the living spermatozoa. 

 I often observed the animals apj^arently in the act of impregnation, 

 but on examining them found no spermatophore. Judging from 

 preserved specimens of the animals and eggs, the spermatozoa of 

 Planocera do not differ from those of Eustylochus. 



The entrance of the spermatozoon into the egg takes place about 

 the time the process of forming the first polar spindle begins, which 

 as I have said occurs either in the ovary itself or, usually at least, 

 before the egg has got far from it. The possibility suggests itself 

 that the entrance of the spermatozoon may be the stimulus which 

 induces the disappearance of the germinal vesicle and the formation 

 of the spindle. 



Another question that arises is why the uni-ipe eggs in the ovaries 

 are not prematurely fertilized. Evidently it is not because the 

 spermatozoa do not have access to the ovaries, although from the 

 numbers of them generally to be seen just outside the ovary, the 

 membrane enclosing the same must serve to keep them out to a con- 

 siderable extent. The most plausible explanation is that the chemi- 

 cal nature of the egg protoplasm is not such as to attract the sper- 

 matozoon until it is of a certain degree of maturity. 



It is exceedingly rare to find more than one spermatozoon in an 

 egg. If double fertilization is prevented by the formation of a 

 vitelline membrane as soon as one spermatozoon has entered, the 

 membrane must be extremely delicate, for I have been unable to 

 distinguish such a structure. If present, it doubtless afterwards 

 forms the inner layer of the tough shell which is deposited around 

 each egg by the shell glands just before laying, and later becomes 

 separated from the egg by a considerable space containing a clear 

 fluid, for if this were not the case we should see it when the polar 

 bodies separate. At that time, however, there is no interior mem- 



