ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 385 
It is notat first easy to say whether the generative elements have their 
proper origin in the ectoderm or endoderm of this body, as in most cases they 
can be merely seen filling the space between these two membranes, which 
become more and more separated from one another as the included mass of 
ova or spermatozoa increases in volume. 
From some favourable observations, however, which I have succeeded in 
making on certain species of hydroids, I have convinced myself that the true 
origin of the ova and spermatozoa is to be found in the endoderm, while the 
ectoderm serves merely as a confining and protecting sac until such time as 
the generative elements acquire sufficient maturity to allow of their liberation, 
which always takes place by simple rupture or absorption of the ectodermal 
sac. 
Thus in the gonophores of the male colonies of Sertularia polyzonias the 
Spermatogenous tissue may be seen filling the entire space between the long 
cylindrical axile spadix and the surrounding walls of the gonophore. In 
most specimens it may be easily seen that the spermatogenous mass is far 
from being of uniform maturity throughout ; for while towards the axis of the 
gonophore it is still very immature, the mother cells being here distinctly 
visible with the ultimate spermatic cells within them, we find that towards 
the periphery it consists of free active spermatozoa. The youngest portion 
of the mass is thus that which is still in contact with the spadix or endoder- 
mal portion of the gonophore, while the oldest portion is situated externally, 
being in contact with the confining ectoderm—a condition which would be 
scarcely possible if the ectoderm, rather than the endoderm, gave origin to the ~ 
spermatic cells. 
A state of things exactly parallel to this may be seen in the female gono- 
phores of Coryne pusilla. At an early period in the development of these 
gonophores, the large thick spadix may be seen to be surrounded by a gra- 
nular plasma, throughout which numerous minute nucleated cells are scattered. 
These cells I regard as the germinal vesicles and spots of the future ova, round 
which no distinctly differentiated vitellus can as yet be detected. In a more 
mature stage of the gonophore, while the same peculiar tissue continues to 
invest the spadix, the peripheral portion of this tissue may be seen to be thrown 
off in the form of undoubted ova, consisting each of a germinal vesicle and 
spot precisely similar to those observed in the more central portion of the 
Mass, but now surrounded by a very definite vitellus. When the gonophore 
has attained complete maturity, the whole of the plasmatic mass has become 
metamorphosed into fully formed ova. 
In Coryne pusilla and many other species, the ova, when escaping 
from the gonophore under the pressure of the compressorium, present a pecu- 
liar appearance. They are then seen to be each invested by a special mem- 
brane of great delicacy, which is continued backwards by a narrow neck-like 
prolongation ; so that in this state the whole ovum presents a pyriform shape. 
This membrane is probably nothing more than the vitellary membrane of the 
ovum, which, from the mode in which the pressure is applied, assumes the 
form described. 
__ Lhave spoken above of an exception to the all but universal fact that the 
generative elements originate between the ectoderm and endoderm of a body 
homologous with the manubrium of a naked-eyed medusa. The exception 
referred to consists in the origination of ova in the blastostyle, as may be 
Seen in Sertularia pumila and one or two other species of Sertularia. 
Tn Sertularia pumila a solitary gonophore of the ordinary form, and con- 
ts in the usual way ova or spermatozoa, originates, as in other cases, by 
863. 2c 
