85 



I did not succeed in following the development farther 

 than to the formation of a gastrula, and was not able to 

 time the changes which were observed, owing to the fact 

 that my watch was broken, I still hope to be able to 

 make a series of observations which will include the later 

 stages. 



Fig. 1 shows an egg which has been re- 

 moved for some time from the ovary ; the 

 vitellus composed of minute corpuscles or 

 spherules has assumed a rounded form, a 

 nucleus is still visible as well as a clear 

 nucleolus within it ; the vitelline mem- 

 brane has become defined, while the deli- 

 cate thin membrane which held the eggs 

 fast to the wall of the ovarian follicle re- 

 Figure 1. mains attached as a transparent structure- 

 less film. It is the suxDposition, based upon the conclu- 

 sions of other observers, that at this stage impregnation 

 has not yet been effected. This is effected by the en- 

 trance of a single spormatozoan or male cell into the egg 

 through the vitelline membrane, but as I have not noted 

 a micropyle, either in the eggs of this animal or that of 

 the oyster, it is very probable that the male cell finds its 

 way into the vitellus through an opening in the vitelline 

 membrane extemporized for its passage. We are, at any 

 rate, not yet ready for a general conclusion as to the 

 presence or absence of a micropyle in the Lamellibranch- 

 iate ovum, to which Balfour in his (Jomparatme Em- 

 bryology^ Vol. I., p. 31, commits himself. The histology 

 of the ovaries of more forms must be studied before we 

 can decide, and no more promising field is open before us 

 than the investigation of the ovogenesis of the oyster and 

 clam, both of which are still unstudied in relation to this 

 point. 



