GENERATIVE ORGANS. 743 



that the endodermal position of the generative organs in the Actinozoa and 

 acraspedote Medusre 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. 



\Ve might perhaps, on this view, formulate the origin of the generative 

 products in the Ccelenterata 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. 



Chaetopoda and Gephyrea. In the Chaetopoda 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 Ccelenterata 1 . 



Chaetognatha. In Sagitta the history of the generative 

 cells, which was first worked out by Kowalevsky and Biitschli, 

 has been recently treated with great detail by O. Hertwig 2 . 



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 ifl my diagram, takes place in such a way that two 



1 The Hertwigs (No. 271) state that in their opinion the generative cells arise 

 from the lining of the body cavity in all the forms whose body cavity is a product of 

 the archenteron. We do not know anything of the embryonic development of the 

 generative organs in the Echinodermata, but the adult position of the generative 

 organs in this group is very unfavourable to the Hertwigs' view. 



2 O. Hertwig, Die Chcetognathen. Jena, 1880- 



