612 



GIIjETOGNATHA. 



Chsetopoda and Gephjrrea. In the Chaetopoda and Gephyrea, 

 the germinal cells are always developed in the adult from the epithe- 

 lial 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. 294 and 295) the 

 generative cells may fairly be held to originate from a layer which 

 corresponds to the endoderm of the Ocelenterata\ 



Ghsetog'natha. 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^ 



The generative cells appear during the gastrula stage, as two 

 large cells with conspicuous nuclei, which are placed in the hypo- 

 blast lining the archenteron, at the pole opposite the blastopore. 

 These cells soon divide, and at the same time pass out of the hypo- 

 blast, and enter the archenteric cavity (fig. 408 A,(je). The division 



Fio, 408. Three stages in the development of Sagitta. (A and C after 



Biitschli, 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 com- 

 mencing to be divided into three. 



C. Represents a later stage, in which the mouth involution (»») has become con- 

 tinuous with the alimentary tract, and the blastopore is closed. 



m. mouth; al. alimentary canal; ae. archenteron; bLp. blastopore; pv. peri- 

 visceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative 

 organs. 



into four cells, which is not satisfactorily represented in my diagram, 

 takes place in such a vfuj 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. 



^ 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 gene- 

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

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



' 0. Hertwig, Die Cfuetognathen. Jena, 1880. 



