XXX VI 
THE VOYAGE OF H.M.S. CHALLENGER 
That form of gonophore which presents the condition of a free Medusa or planoblast, 
has like the sedentary gonophores the office of bringing to maturity the sexual elements. 
In almost all these, however, the sexual products arise within the Medusa, and do not 
therefore need to migrate from a distance into their place of maturation. 
In those planoblasts whose generative elements are borne along the course of the 
radiating canals (Leptomedusse) that part of the walls of the canal in which these 
are contained usually projects with the included ova or spermatozoa into the cavity of 
the umbrella in the form of a pouch or gonad, which has often the appearance of a dis- 
tinct gonophore budded off as an independent zooid from the canal. I formerly regarded 
it as such and made this view the grounds for separating the planoblasts into such as 
give origin to their sexual elements directly in the walls of the manubrium (gonochemes 
= Anthomedusse), and such as produce them only through the intervention of a special 
sexual bud (blastochemes = Leptomedus8e). I now, however, believe it more correct to 
regard the sexual pouches or gonads which are produced in the course of the radiating 
canals, not as true buds, but as simple extensions of the canal walls, caused by the 
presence of the sexual products between their endoderm and ectoderm ; and I would 
accordingly view this second form of planoblast as also giving rise to its sexual elements 
directly, and therefore, like the first, as the exact locomotive equivalent of the hedrio- 
blast or sedentary gonophore. 
3. Development oj- the Ovum. 
In by far the greater number of species the development of the ovum results in the 
formation of a ciliated locomotive larva known as a planula. In such cases the ovum, 
which is mostly destitute of vitellary membrane, after passing through a regular or 
nearly regular segmentation in accordance with the usual binary law of embryonal 
development, becomes transformed into a solid spherical mass of cells (blastosphere) from 
which a peripheral layer soon becomes separated by a process of delamination. The 
embryo now as a rule becomes more or less elongated, and a central cavity makes its 
appearance in it. 
At this stage the embryo is in the form of a hollow oviform body whose walls are 
composed of two layers, an external or ectoderm, and an internal or endoderm. It is 
by delamination, never by invagination, that the two germinal layers, ectoderm and 
endoderm, are formed. The embryo has now usually escaped from the confinement of 
the gonophore, and its ectoderm becomes clothed with vibratile cilia, by the aid of which 
wish for an opportunity of examining other examples which might supplement the very scanty material at the disposal 
of Goette. 
In some common Hydroids no gonosome has yet been found. Until, however, we know the time of year and other 
conditions which may he here necessary for the development of the gonosome, an ignorance of this element in certain 
Hydroids affords no grounds for regarding its absence in these as a constant and permanent occurrence. 
