UKMTAL 



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 epi- 

 thelium 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 Nematodes. 

 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, aud 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 primi- 

 tive type of echinoderm generative organ consists of a blind sack, 

 projecting into the body cavity, and opening by a pore to the exterior. 

 The sack is lined by an epithelium, continuous with the epidermis, the 

 cells of which give rise to the ova or spermatozoa. The duct of these 

 organs is obviously hardly differentiated from the gland ; and the 

 whole structure might easily be derived from the type of generative 

 organ characteristic of the Hydromedusa?, where the generative cells 

 are developed from special areas of the ectoderm, and, when jipe, pass 

 directly into the surrounding medium. 



If this suggestion is correct we may suppose that the genera- 

 tive ducts of the Echinodermata have a different origin to those of the 

 majority of 1 the remaining tiiploblastica 



Their ducts have been evolved in forms in which the generative 

 products continued to be liberated directly to the exterior, as in the 

 Hydro medusae; while those of other types have been evolved in forms 

 in which the generative products were first transported, as in the 

 Actinozoa, into the gastrovascular canals 2 . 



: It would be interesting to have further information ahout Balanoglossus. 

 - These views fit in very well with those aheady put foiward in Chapter xin. on 

 the affinities of the Echinodermata. 



