(2) The generative products of both sexes originate in the entoderm (hypoblast): Plumularia and Sertularella, amongst the Hydroids, and the whole of the Acraspeda and Actinozoa.
(3) The male cells are formed in the ectoderm, and the female in the entoderm: Gonothyræa, Campanularia, Hydractinia, Clava.
In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Cœlenterata have led, it would seem to be necessary either to hold that there is no definite homology between the germinal layers in the different forms of Cœlenterata, or to offer some satisfactory explanation of the behaviour of the genital products, which would not involve the acceptance of the first alternative.
Though it can hardly be said that such an explanation has yet been offered, some observations of Kleinenberg (No. [557]) undoubtedly point to such an explanation being possible.
Kleinenberg has shewn that in Eudendrium the ova migrate freely from the ectoderm into the endoderm, and vice versa; but he has given strong grounds for thinking that they originate in the ectoderm. He has further shewn that the migration in this type is by no means an isolated phenomenon.
Since it is usually only possible to recognise generative elements after they have advanced considerably in development, the mere position of a generative cell, when first observed, can afford, after what Kleinenberg has shewn, no absolute proof of its origin. Thus it is quite possible that there is really only one type of origin for the generative cells in the Cœlenterata.
Kleinenberg has given reasons for thinking that the migration of the ova into the entoderm may have a nutritive object. If this be so, and there are numerous facts which shew that the position of generative cells is often largely influenced by their nutritive requirements, it seems not impossible that the endodermal position of the generative organs in the Actinozoa and acraspedote Medusæ may have arisen by a continuously earlier migration of the generative cells from the ectoderm into the endoderm; and that the migration may now take place at so early a period of the development, that we should be justified in formally holding the generative products to be endodermal in origin.
We might perhaps, on this view, formulate the origin of the generative products in the Cœlenterata in the following way:—
Both ova and spermatozoa primitively originated in the ectoderm, but in order to secure a more complete nutrition the cells which give rise to them exhibit in certain groups a tendency to migrate into the endoderm. This migration, which may concern the generative cells of one or of both the sexes, takes place in some cases after the generative cells have become recognisable as such, and very probably in other cases at so early a period that it is impossible to distinguish the generative cells from indifferent embryonic cells.
Very little is known with reference to the origin of the generative cells in the triploblastic Invertebrata.