Sexual Characteristics of Oysters. 43 
Examining sections, however, we never find either in the 
reproductive follicles of O. virginica or of O. angulata any 
evidence of the coexistence of ovules and spermatozoa. In 
fact the mode of spermatogenesis in the unisexual species is 
very different from that of the hermaphroditic. As indicated 
in Brooks’s figure of a part of a section of a male oyster, the 
spermatozoa are peculiarly arranged in the follicle or tubule. 
Upon applying a high power (500 to 800 diameters) I find 
that the heads of the spermatozoa show a very marked 
tendency to be arranged in rows like beads, and not in oblong 
clusters as in the hermaphroditic species. Moreover the walls 
of the generative tubules are lined by relatively very much 
smaller spermatoblasts than those found free in the repro- 
ductive follicles of the hermaphrodite form. ‘This spermato- 
genetic layer is often very marked in the males of the uni- 
sexual species, and even at an early stage of the functional 
activity of the testicular organ presents much the same struc- 
ture that it does later. ‘The rows of spermatozoa already 
alluded to also have a tendency to be bent towards the outlet 
ot the tubules, giving rise to a fringe-like appearance on either 
side of the follicle with a clearer space between the edges of the 
fringe-like masses of spermatozoa. In fact it is plainly to be 
seen that the spermatozoa are being budded off from the 
spermatogenetic layer, and that the appearances just described 
are a result of that process. It results from this that the 
structural peculiarities of the testicular tubules are very cha- 
racteristic, so that once recognized they will never afterwards 
be confounded with the arrangement observed in the ovary of 
the female, where, as in the hermaphrodite species, the ova 
may be seen in different stages of development, though, where 
the majority of the ovules have attained nearly full develop- 
ment, it may happen that few of the nascent ovules closely 
adherent to the walls of the follicles are visible. 
The distinction between Ostrea edulis and the American 
and Portuguese species is therefore very marked and impor- 
tant. Mdébius(‘Die Auster unddie Austernwirthschaft,’ Berlin, 
1877, p. 19) says of their species: —Oysters are hermaphro- 
dites. In the largest number of individuals, in the whole repro- 
ductive organ | found only spermatozoa, but no eggs. In 
seven oysters which carried blue brood in the beard, the sexual 
gland contained only spermatozoa. Three oysters with younger 
white embryos in the beard had no spermatozoa in the sexual 
gland. Inthe most of the brood-bearing oysters the sexual 
gland contained neither eggs nor spermatozoa. Of 309 oysters 
which were taken on the 25th of May from four different 
banks east of the island of Sylt, and afterward examined 
