752 DERIVATION FROM EXCRETORY ORGANS. 



the Insecta appears to point to their having originated independ- 

 ently of the excretory organs. 



In the Nematoda the generative organs of both sexes 

 originate from a single cell (Schneider, Vol. I. No. 390). 



This cell elongates and its nuclei multiply. After assuming 

 a somewhat columnar form, it divides into (i) a superficial 

 investing layer, and (2) an axial portion. 



In the female the superficial layer is only developed distinctly 

 in the median part of the column. In the course of the further 

 development the two ends of the column become the blind ends 

 of the ovary, and the axial tissue they contain forms the 

 germinal tissue of nucleated protoplasm. The superficial layer 

 gives rise to the epithelium of the uterus and oviduct. The 

 germinal tissue, which is originally continuous, is interrupted in 

 the middle part (where the superficial layer gives rise to the 

 uterus and oviduct), and is confined to the two blind extremities 

 of the tube. 



In the male the superficial layer, which gives rise to the 

 epithelium of the vas deferens, is only formed at the hinder end 

 of the original column. In other respects the development takes 

 place as in the female. 



In the Insecta again the evidence, though somewhat conflicting, 

 indicates that the generative ducts arise very much as in Nema- 

 todes, from the same primitive mass as the generative organs. In 

 both of these types it would seem probable that the generative 

 organs were primitively placed in the body cavity, and attached 

 to the epidermis, through a pore in which their products passed 

 out ; and that, acquiring a tubular form, the peripheral part of 

 the gland gave rise to a duct, the remainder constituting the true 

 generative gland. It is quite possible that the generative ducts 

 of such forms as the Platyelminthes may have had a similar 

 origin to those in Insecta and Nematoda, but from the analogy 

 of the Mollusca there is nearly as much to be said for regarding 

 them as modified excretory organs. 



In the Echinodermata nothing is unfortunately known as to 

 the ontogeny of the generative organs and ducts. The structure 

 of these organs in the adult would however seem to indicate that 

 the most primitive type of echinoderm generative organ consists 

 of a blind sack, projecting into the body cavity, and opening by 



