MORPHOLOGY. 



9. The Generailve Mements. 



I have thus far endeavoured to give a complete account of the morphology of those parts 

 which are destined for the origination and protection of the generative elements. These elements 

 themselves maj' next be examined. 



The existence of generative elements — ova and spermatozoa — has now been fully determined 

 in every important group of the Hydroida. 



Ova. — The hydroid ovum (woodcut, fig. 40 A), in all those cases where its structure has been 

 satisfactorily seen, consists of a granular vitellus enveloping (except in the genus TubuJaria and 

 probably the other ac//;2«/a-producing hydroids) a distinct, more or less excentric, germinal vesicle, 

 in which one or more germinal spots may be almost always demonstrated, and occasionally with one 

 or more puncta or nucleoli in the interior of the germinal spot. The whole is invested by an ex- 

 ceedingly delicate vitellary membrane, which, though it sometimes escapes detection, is probably 

 always present, at least during some period in the existence of the ovum. In the genus T/ihuIaria 

 the most careful investigation has as yet failed in detecting any trace of germinal vesicle or 

 spot. 



From some observations which I have been enabled to make on certain very earl)' stages of 

 the ovum, it would seem that the germinal vesicle shows itself before any distinctly differentiated 

 vitellus has begun to envelope it, and that the vitellus afterwards accumulates round the germinal 

 vesicle as round a separate centre of differentiation. (See below, where this process is more fully 

 described in the physiological section.) 



In Conjne inisilla and many other species, the ova, when escaping from the gonophore under 

 the pressure of the compressorium, present a peculiar ajipearance. They are then seen to be 

 each invested by a special membrane 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. 



In no hydroid ovum have I found any evidence of a raicropyle. 



Spermatozoa. — The spermatozoa possess the form which so generally characterises those bodies 

 throughout the animal kingdom, being here in all cases active caudate corpuscles (woodcut, fig. 31 

 D d). The caudal filament is sometimes of such extreme tenuity as to render it very difficult of 

 detection, while the head varies in form, being usually conical — and then with the filament 

 attached to the wide end of the cone — but sometimes spherical, or cylindrical, or " guitar-shaped,"' 

 according to the species. In Eudendrium ramosum a very minute granule may always be seen 

 attached to one side of the head of the spermatozoon, where it looks like a parietal nucleus. 

 (PI. XIII, fig. 17.) 



The spermatozoa seem to be always developed in true sperm-cells, which are themselves 



^ The spermatozoa of Eudendrium dispar, Agass., and some other species, are so described by 

 Agassiz. • Nat. Hist. United States,' vol. iv. 



