Sexual Characteristics of Oysters. 45 
ments as follows :—“ In appearance they were quite similar to 
the American species (Ostrea virginica), having long shells of 
from 1 to 3 inches in length, rougher and thicker than is 
usually the case with the Huropean oyster.’ This remark 
raises the question whether the experimenter was not really 
working with O. angulata instead of O. edulis. The locality 
where he got his specimens and where he conducted his expe- 
riments also makes it not improbable that he was in reality 
working with the native unisexual species, O. angulata. 
To return to the question of the breeding-habits of Osérea 
edulis, it appears to me that we cannot very well question the 
authority of Mobius, Lacaze-Duthiers, and Horst, in regard 
to the bisexual state of the reproductive organs. My investiga- 
tions also give some countenance to the fact of a preponderance 
either of eggs or of spermatozoa in different individuals ; in fact, 
in some cases the one or the other seems to be almost exclusively 
the mature product. But we are not yet in a position to 
arrive at a conclusion in this matter, because of the scantiness 
of the observations which have hitherto been made. The 
hypothesis that the spermatozoa are drawn from without into 
the generative ducts by the ciliary action of the gills and 
mantle may be dismissed with the remark that microscopic 
investigation, to my mind, has effectually disposed of the pro- 
bability of any such a state of affairs. We may see the sper- 
matozoa in course of development in the same follicle with the 
ova, which is conclusive proof that the milt has not been de- 
rived from without, from the water into which it had been 
discharged by neighbouring individuals. In truth, we find in 
some cases the spermatozoa present so deep down in the 
utmost ramifications of the generative follicles that it is not 
conceivable that they should have been drawn in from 
without. 
As to the alternate activity of the organs in producing ova 
and spermatozoa, there is a possibility that such is the case, 
but, as stated at the outset, there is as yet no conclusive proof 
of the fact. Certain it is that I have yet to see sections of 
O. edulis in which both ova and spermatozoa are not present 
in some condition of development at the same time. If the 
one be not present in a fully developed state, developing traces 
of it may be discovered; or even a very minute quantity of 
developed milt or a few developed eggs may be present in 
some one follicle, while in the others there are perhaps exclu- 
sively eggs or exclusively milt in a developed condition. I 
am aware that this view of the matter is opposed to the 
current doctrine that nature provides against continuous inter- 
breeding ; but when we find the eggs and milt about equally 
