42 COMPARATIVE ANATOMY 



of a single nephridium may be described as follows : — An indi- 

 vidual nephric cell increases in size, becomes detached from the 

 nephric row and takes up a position among the cells of a 

 septum separating two somites. It then divides into two cells, 

 a larger lying partly imbedded in, partly projecting in front of, 

 the septum, and a smaller projecting behind the nephridium. 

 The large anterior cell gives rise to the nephrostome, which is 

 at first very simple, consisting of two cells enclosing a cavity, 

 within which is a single flagellum. The small hinder cell by 

 repeated transverse division gives rise to a row of cells, which 

 grows towards, and eventually becomes attached by one end to, 

 the ventral epiblast. The cells of this row are at first solid, 

 but eventually they become hollow, and the row, greatly in- 

 creased in length and thrown into convolutions as the result of 

 a lateral outgrowth, the details of which need not detain us, 

 develops into the ciliated and glandular parts of the nephridial 

 tube. The muscular section of the nephridium is formed from 

 an invagination of the epiblast. By a further process of cell 

 division and growth the simple embryonic nephrostome becomes 

 converted into the complicated nephrostome of the adult. 

 Nephridia are not formed in the first pair of somites. It 

 appears from this that the nephridia are derived from the epi- 

 blast, and come into secondary relations with the cavities 

 hollowed out in the mesoblastic somites. 



On the other hand, the sperm-ducts and oviducts appear to 

 be formed as outgrowths of the ccelomic epithelium. These 

 outgrowths penetrate the body-wall and acquire openings to 

 the exterior, the male and female generative pores. Thus, 

 although in position and structure the generative ducts offer a 

 strong resemblance to nephridia — they are developmentally 

 quite distinct from them. The nephridia are epiblastic struc- 

 tures which grow towards the mesoblast, the gonaducts are 

 mesoblastic structures which grow outwards towards the meso- 

 blast. Since the two sets of organs are not homologous, it is 

 advisable to reserve the name peritoneal funnel exclusively for 

 structures which, like the gonaducts, are derived from the 

 mesoblast, and grow outwards, and acquire external openings. 



To return to the mesoblast bands. It has been shown that 

 at a comparatively early stage the two bands meet and unite 

 anteriorly dorsal to the persistent anterior part of the blasto- 

 pore which becomes the mouth. Whilst the mesoblastic 



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