382 REPORT— 1863. 
Among the Campanularians we also meet with many examples of free 
medusa-buds, as well as of fixed sporosacs, the free meduse being, so far as 
we yet know, rather more frequent than the sporosacs. They are both borne 
as buds upon the blastostyle of a gonangium ; but the free meduse are (with 
two known exceptions, those, namely, which are afforded by Laomedea tenuis, 
mihi, and Lafewa cornuta, Lamx.) always gonoblastochemes, the generative 
elements being produced in special sexual buds which arise from some part 
of the radiating canals. 
Finally, among the Sertularians we know as yet of no instance of a free 
medusa, the generative elements being among these hydroids always produced 
in adelocodonic gonophores which, as in the Campanularians, are invariably 
borne upon the blastostyle of a gonangium*. 
III. Srructvre anp Formation oF THE GENERATIVE ELEMENTS, 
The existence of generative elements—ova and spermatozoa—has now been 
fully determined in every important group of the Hyproma. 
Ova.—The hydroid ovum (fig. 20 A), in all those cases where its structure 
has been satisfactorily seen, consists of a granular yitellus enveloping (except 
alone in the genus 7'ubularia) 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 nucleoli in the interior of the germinal spot. 
The whole is invested by an exceedingly delicate vitellary membrane, which, 
though it sometimes escapes detection, is probably always present, at least in 
the young ovum. In the genus 7ubularia alone the most careful investiga- 
tion has as yet failed in detecting any trace of germinal vesicle or spot. 
In the earliest stage in which I have in any case succeeded in observing 
the ova, that namely presented by these bodies in the young gonophore of 
Coryne pusilla, I have found nothing but minute, clear, nucleated vesicles 
immersed in a common granular plasma. That these are to constitute the 
germinal vesicle and spot of the more mature ovum seems certain, while the 
granular plasma in which they are immersed would appear to represent the 
vitellus, though I could as yet trace no differentiation in it indicating a 
separation into distinct masses accumulated round the individual germinal 
vesicles. Ina more mature stage, however, each vesicle had its own special- 
ized vitellus surrounded by a distinct vitellary membrane. 
Spermatozoa.—The spermatozoa possess the form which so generally cha- 
racterizes those bodies throughout the animal kingdom, being here in all 
cases active caudate corpuscles (fig. 14 Dd). The caudal filament is some- 
times of such extreme tenuity as to render it very difficult of detection, 
sacs, while he has never observed generative elements in the manubrium. I feel convinced 
that the doubts of Agassiz on this point are even better founded than he himself will admit, 
and that the structures alluded to have nothing to do with generative sacs; while the 
analogy of closely allied species (see Cavolini, Mem. Polypi Marini; MeCrady, Gymnoph- 
thalmia of Charleston Harbour) renders it almost certain that Pennaria gibbosa affords no 
exception to the general rule, that the free meduse of the Tudularida never develope 
generative buds upon the course of the radiating canals. 
* Agassiz (op. cit. vol. iii. pp. 46 & 48), referring to a Sertularian which he names 
Dynanena Fabricti, and calls one of the most common Sertularians of the Atlantic coast 
of North America, asserts that this hydroid produces free meduse, of which he gives 
figures. There is probably some mistake here; the fact is mentioned only in a casual 
way ; and in the following volume, in which a detailed account of the North American 
species of Hydroida is given, no allusion is made to it, though the closely allied, if not 
identical, Dynamena (Sertularia) pumila is minutely described. ‘ 
