Chætopoda and Gephyrea. In the Chætopoda and Gephyrea, the germinal cells are always developed in the adult from the epithelial lining of the body cavity; so that their origin from the mesoblast seems fairly established.
If we are justified in holding the body cavity of these forms to be a derivative of the primitive archenteron (vide pp. [356] and [357]) the generative cells may fairly be held to originate from a layer which corresponds to the endoderm of the Cœlenterata[269].
Chætognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Bütschli, has been recently treated with great detail by O. Hertwig[270].
The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore. These cells soon divide, and at the same time pass out of the hypoblast, and enter the archenteric cavity ([fig. 408] A, ge). The division into four cells, which is not satisfactorily represented in my diagram, takes place in such a way that two cells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the ovaries, one half of each primitive cell thus forming an ovary, and the other a testis.
Fig. 408. Three stages in the development of Sagitta. (A and C after Bütschli, and B after Kowalevsky.)
The three embryos are represented in the same positions.
A. Represents the gastrula stage.
B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.
C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore is closed.
m. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. perivisceral cavity; sp. splanchnic mesoblast; so. somatic mesoblast; ge. generative organs.
Fig. 409. Two views of a late embryo of Sagitta. A, from the dorsal surface. B, from the side. (After Bütschli.)
m. mouth; al. alimentary canal; v.g. ventral ganglion (thickening of epiblast); ep. epiblast; c.pv. cephalic section of body cavity; so. somatopleure; sp. splanchnopleure; ge. generative organs.