ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 409 
phores which contain it a position precisely similar to that held by the un- 
doubted spermatogenous tissue in the male gonophores of the same species ; 
and as nothing else is presented by the hydroid which can in any way be 
regarded as ova, we should, by denying to these the essential attributes of 
ova, be reduced to the anomalous alternative of admitting the existence of 
the male element without the correlative female one. 
The phenomena connected with the development of Tubularia indivisa, in 
which I have carefully examined them *, will afford a good example of the 
difference between this form of development and that which is usual among 
the Hydroida; in all essential points they are the same as in the other 
species of Tubularia. 
In the female gonophore of Tubularia indivisa (fig. 6 C), the generative pro- 
duct originates as a voluminous plasma between the endoderm and ectoderm 
of the manubrium. It is evidently in more intimate relation with the endo- 
derm than with the ectoderm, and as it increases in bulk it would seem to cause 
the absorption of the latter membrane, which had confined it in its young 
state. A portion of it now becomes detached from the mass, and soon under- 
goes a special development into an embryo within the cavity of the gono- 
phore. As has just been said, no trace of germinal vesicle or spot can be 
found either in the entire mass or in any of the detached portions; so also 
the phenomenon of yolk-cleavage, if present at all, is very obscure, but the 
detached mass may be easily broken up into cells filled with secondary 
cells. 
The ovum (for I have no hesitation in so designating the mass detached 
from the primitive plasma, notwithstanding its anomalous characters) lies in 
contact with the remainder of the plasma, and while in this position becomes 
developed into an actiniform embryo, as has been already noticed by Van 
Benedent, Mummery?t, and others. In the act of development it becomes 
first extended as a disc over the residual plasma. In this disc we can always 
recognize a differentiation between its peripheral and central portions. Next, 
from the circumference of the disc short and thick processes radiate all 
round, and these soon elongate themselves into tentacles; the disc at the 
same time gradually becomes more gibbous on the side turned away from the 
axis of the gonophore, its interior becomes hollowed out into a digestive 
cavity, and a mouth makes its appearance in the centre of the opposite side, or 
that in contact with the plasma. The embryo now retreats from the plasma, 
the mouth is seen to be elevated on a conical prominence, while the side 
opposite to the mouth becomes more and more prolonged until it assumes the 
form of an elongated oval peduncle into which the general cavity is continued. 
The extremity of this prolongation presents the appearance of delicate striae 
(probably fibres) radiating for a short distance from its central point—a pe- 
culiar structure which might easily lead to the belief that an aperture was 
here present. The «wppearance of an aperture, however, I believe to be entirely 
deceptive. At the same time a circle of very short tentacula makes its ap- 
pearance immediately around the mouth. In this state it escapes from the 
gonophore, and, after continuing free for a period, the side opposite to the 
mouth becomes ultimately developed into a cylindrical stem, which soon 
clothes itself with a periderm and fixes the young Zubularia to some neigh- 
bouring object. After the escape of the embryo, or even during its develop- 
* “Notes on the Hydroid Zoophytes,” Ann. Nat. Hist. July 1859. 
t “Recherches sur, l’Embryogénie des Tubulaires,” p. 37. pl. 1, in Nouv. Mém. de 
PAcad. Roy. de Bruxelles, tom. xvii. 1844. 
¢ “On the Development of Tubularia indivisa,’ Trans. Micr. Soc. 1853, p. 28. 
