When the archenteric cavity is divided into a median alimentary tract, and two lateral sections forming the body cavity, the generative organs are placed in the common vestibule into which both the body cavity and alimentary cavity at first open ([fig. 408]).
The generative organs long retain their character as simple cells. Eventually ([fig. 409]) the two ovaries travel forwards, and apply themselves to the body walls, while the two testes also become separated by a backward prolongation of the median alimentary tract.
On the formation of the transverse septum dividing the tail from the body, the ovarian cells lie immediately in front of this septum, and the testicular cells in the region behind it.
Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek finds that the generative organs originate from a pair of specially large mesoblast cells, situated in the space between the stomach and the floor of the vestibule. The two cells undergo changes, which have an obvious resemblance to those of the generative cells of the Chætognatha. They become surrounded by an investment of mesoblast cells, and divide so as to form two masses. Each of these masses at a later period separates into an anterior and a posterior part. The former becomes the ovary, the latter the testis.
Nematoda. In the Nematoda the generative organs are derived from the division of a single cell which would appear to be mesoblastic[271].
Insecta. The generative cells have been observed at a very early embryonic stage in several insect forms (Vol. II. p. 404), but the observations so far recorded with reference to them do not enable us to determine with certainty from which of the germinal layers they are derived.
Crustacea. In Moina, one of the Cladocera, Grobben[272] has shewn that the generative organs are derived from a single cell, which becomes differentiated during the segmentation. This cell, which is in close contiguity with the cells from which both the mesoblast and hypoblast originate, subsequently divides; but at the gastrula stage, and after the mesoblast has become formed, the cells it gives rise to are enclosed in the epiblast, and do not migrate inwards till a later stage. The products of the division of the generative cell subsequently divide into two masses. It is not possible to assign the generative cell of Moina to a definite germinal layer. Grobben, however, thinks that it originates from the division of a cell, the remainder of which gives rise to the hypoblast.
Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery ([figs. 405] C and [410], po). They appear to be derived from the epithelial cells amongst which they lie; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium.
