1873.] 175 [McCrady. 



The egg of the oyster, therefore, appears to pass through the fol- 

 lowing phases. 



1. A phase in which there are only germinative vesicles embedded 

 in a common mass of yolk, exhibiting no differentiation into distinct 

 yolk masses. 



2. A later phase, in which each germinative vesicle has appropri- 

 ated its proportionate share of yolk, which gradually becomes sur- 

 rounded with a vitelline membrane. 



3. A phase still later, in which each complete ovum may consist of 

 ectoblast, mesoblast, entoblast, and entosthoblast, and is lodged in a 

 distinct pear-shaped ovisac, attached by its narrow and rather elon- 

 gated neck to an ovarian membrane. 



4. Spermatozoa have been observed to be present in the ovary at 

 every one of these stages, without the appearance of any of the 

 phenomena of impregnation. It must, however, be here remarked 

 that the only observation of the spermatozoa in the ovisac phase was 

 in a specimen in which there was neither nucleus nor nucleolus re- 

 corded as observed in the germinative vesicles. 



It is evident that the evolution of perfect spermatozoa antedates 

 that of perfect ova, though these former may continue to be seen in 

 the reproductive organ up to the time when the ova have nearly 

 reached maturity. 



The spermatozoa may be seen in an active state when the ova are 

 as yet scarcely determinable. Again, we cannot affirm that at the 

 stage of actual maturity of the ovum, any spermatozoa are present. 



The case is still further complicated by the fact that in the climate 

 of Charleston the spawning time of the oyster seems to extend from 

 May to November, the spawning being heaviest from about the mid- 

 dle of June to the middle of September. At almost any time during 

 the summer it is possible to find individuals with very immature ova. 



It remains a question, therefore, whether self-impregnation is not 

 actually barred by some not yet known provision, and whether the 

 spermatozoa are not freely discharged into the water to be wafted to 

 other individuals, whose ova may be ready for fertilization. The 

 closely gregarious and crowding habits of the oyster render this alto- 

 gether possible, and in cases where the sexual products have flowed 

 from a tumid generative organ on pressure by the finger, I have ob- 

 served at one time the extrusion of ova only, at another, the extru- 

 sion of spermatozoa only. 



