McCrady.] 174 [Dec. 3, 



the rate of nineteen or twenty pulsations a minute. This is much 

 more than the rate of beating observed in oysters which have been 

 some time out of the water, in which there are long intervals of rest 

 between the beats. 



" The examination of the ovary proved very interesting, for I 

 found numerous ova, all provided not only with germinative vesicle, 

 but with one or more Wagnerian vesicles, and these often containing a 

 dot. This is quite different from Davaine's results, as he never was 

 able to establish the existence even of the Wagnerian vesicle at any 

 point of the growth. Moreover, the ova were lodged in distinct 

 pyriform sacs, precisely as is usually the case in other Lamellibranchi- 

 ates, and of this also Davaine makes no mention. The shape of 

 these sacs was precisely that of a Florence flask, and the neck quite 

 long. One view in profile was accidentally obtained, showing clus- 

 ters of these pyriform ovisacs standing out from the membrane of the 

 ovary. 



" It is quite impossible that there should be any error as to the 

 existence in this case of the Wagnerian vesicles, as they were much 

 larger than the yolk cells, and of a quite different appearance, being 

 seemingly mere specializations of the substance of the germinative 

 vesicle, and like it quite transparent. The dot within these Wagne- 

 rian vesicles appeared to be a congeries of granules. 



" I endeavored next to ascertain whether or not spermatozoa were 

 present, but could not satisfy myself on this point, as my eye had 

 become fatigued, and no disposition I could make of the light enabled 

 me to discover whether the minute dancing cellules, which were quite 

 numerous, had or had not a tail." 



This observation makes it evident that the egg of the oyster does 

 pass through a stage in which it has an ectoblast, mesoblast, ento- 

 blast, and entosthoblast, like other ova, and that it also, like the ova 

 of other Lamellibranchiates, has a stage in which it is lodged in a 

 distinct pyriform saccular diverticulum of an ovarian membrane. 

 This latter point was confirmed by several other recorded observa- 

 tions, in some of which the sac existed, though the germinative vesi- 

 cle contained neither nucleus nor nucleolus. 



I am therefore led to doubt the conclusion arrived at by Agassiz in 

 the case of Tubularia (Parypha) cristata, where there is a similar 

 obscurity, that in that case the ovum passes into the embryo without 

 the intermediation of a stage characterized by the ordinary complica- 

 tion of structure. 



