1873.] 173 [McCrady. 



both of these advanced conditions, as well as in that which Davaine 

 observed; and there is also recorded among my notes an observation 

 upon an old oyster in which, notwithstanding the absence of any 

 observable germinative vesicles, the yolk had undergone its regular 

 segmentation into distinct yolk masses; while a young oyster exam- 

 ined at the same time contained similar perfectly separated yolk 

 masses, each surrounded by a vitelline membrane, and containing a 

 very distinct simple germinative vesicle (but without nucleus or nucle- 

 olus), easily separated from the yolk. This observation may mean 

 that oysters become sterile so far at least as the female products of 

 the generative organs are concerned, with advancing age. 1 



In reference to these more advanced stages of the egg in the oys- 

 ter, not hitherto observed, so far as I am aware, by any one, I prefer 

 to give an extract from my journal of observations written at the time 

 of observation. 



"Examined to-day (April 23, 1869) a good-sized mill-pond oys- 

 ter. It was quite lean, and the shell whitish, but it had the black 

 mantle and the yellow color. It was opened immediately after being 

 taken out of the water of my aquarium and the heart was beating at 



Fig. 1. 



Egg of O. virginiana in various stages. 

 a, germinative vesicles, y, yolk, c, egg-capsule. 



*Tn such a case we may, however, suspect the possible existence of a Partheno- 

 genesis. There is no known reason why yolk-mass should not in all cases be germ- 

 mass, and of the nature of a bud, capable under favorable circumstances of 

 developing into a new individual ; but ordinarily overcome and assimilated as food 

 by the superior vitality of the embryo, resulting from the contact of the sperma- 

 tozoa with the germinative vesicle. This seems to me the direction in which we 

 should look for the explanation of such Parthenogenesis as appears to exist in the 

 female Bee. 



