DEVELOPMENT. 91 



the earlier stages of their development. The plasma in which the}' originate holds in the gono- 

 phores which contain it a position precisely similar to that held by the nndonbted spermatogcnons 

 tissue in the male gonophores (figs. 8/, 9/, and 21/) 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 clement without the correlative female one. 



The fact, however, that the plasma at a very early period, as well as the masses which have 

 been detached from it in order to become developed into embryos, consist almost entirely of cell- 

 like elements, indicates a difference between the matter composing them and the more simple 

 protoplasmic matter of the unfecundated vitellus in other hydroids. 



The phenomena connected with the development of the embryo in Ttibularia indivisa and 

 T. larynx, in both of 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 ; they 

 would seem to be in all essential points similar in the other species of Tuhularia. 



In the very young female gonophore of Tubniaria larynx, while yet only ^^th of an inch 

 in diameter, the spadix may be seen lying in the axis of a cavity bounded externally by a double 

 wall (PI. XXIII, fig. 19). Surrounding the spadix, and occupying the whole of the space between 

 it and the wall of the cavity, may be seen the generative plasma (/), consisting of a uniform mass 

 of small spherical cells, about 2-oooth of an inch in diameter. When liberated from the young 

 gonophore, and floated in water, these cells seem perfectly transparent, their contents appearing to 

 consist of a clear colourless fluid, with a somewhat higher refractive power than the surrounding 

 water. Under the action of acetic acid their contents become granular, and a nucleus-like 

 particle usually becomes visible in the midst of the granular contents (fig. 20). 



At a slightly more advanced stage (fig. 21) the gonophore has reached to about xko^ of an 

 inch in diameter, and the apical tubercles (a') which characterise the mature gonophore have 

 begun to make their appearance. The inner layer (i) of the walls of the gonophore may now be 

 seen to have become separated from the outer («), and thereby i-endered more distinct. This 

 inner laj'er is plainly composed of minute spherical cells, and is thinner than the outer wall, 

 which is composed of prismatic cells, among which thread-cells are already developed. At this 

 period the gonophore begins to become perforated at its summit by an aperture, which opens 

 externally between the bases of the apical tubercles. 



The tubercles continue to increase in size with the enlarging gonophore ; the plasma becomes 

 more voluminous, and among its component cells may be seen several of somewhat larger size 

 than the rest ; under the action of acetic acid these larger cells show a very distinct nucleus, with 

 nucleolus, in the midst of granular contents (fig. 22). It is just possible that these cells may 

 represent germinal vesicles with the germinal spot and its contained ptmctum, but with no vitellus 

 as yet differentiated around them. The plasma, retaining the same structure, continues to 

 increase in volume with the growth of the gonophore ; while the inner layer of the wall — that 

 which had immediately invested the plasma, and must be regarded as the endotheca — would 

 seem to undergo absoi-}3tion, and finally to disappear. We now find that a portion (fig. 23 y) 

 of the plasma has become detached from the mass, and soon undergoes a special development into 

 an embryo within the cavity of the gonophore. As has been said, no obvious trace of germinal 



' "Notes ou the Hydroid Zoophytes," 'Auu. Xat. Hist.,' July, IS.I'J. 



