iv OPISTHONEPHROS 247 



becomes constricted off as a small round vesicle with thick wall, 

 composed of tall epithelial cells, and a small lumen. This is a 

 secondary nephrotome. It remains for a time without change hut 

 eventually behaves very much as the original (primary) nephrotome, 

 one wall becoming invaginated to form a glomerulus, and pocket-like 

 outgrowths giving rise, one to a tubule rudiment, the other to a 

 peritoneal canal. An important difference in detail is seen in the 

 behaviour of the duct, which sends out a tubular projection of con- 

 siderable length to meet the secondary tubule. ' This outgrowth 

 arises from the duct some distance behind the point where the 

 primary tubule opens into it. 



The secondary nephrotome in turn buds off a tertiary nephrotome 

 which again behaves as before and its tubule is met by a projection 

 from near the tip of the outgrowth of the duct which has already 

 developed in relation to the secondary nephrotome. Consequently 

 secondary and tertiary tubules open into the archinephric duct by a 

 common collecting-tube formed of this outgrowth. 



Apparently new generations of subsequent nephrotomes may go 

 on being formed in a similar fashion each from the preceding one 

 until there may be as many as eight in a single segment, all of them, 

 except the primary, opening into a common collecting-tube. 



The degree of development reached by the opisthonephric units 

 is different in different parts of its length. They attain full develop- 

 ment in the manner above described in the region of segments 50- 

 100. In the region in front of this (segts. 30-50) the secondary 

 nephrotomes and their derivatives never become functional and their 

 rudiments degenerate. Still further forward (segts. 24-29) even the 

 primary units as a rule degenerate without completing their develop- 

 ment. 



Apart from differences in detail it is clear that the primary units 

 of the opisthonephros present the most striking resemblance to those 

 of the pronephros and the evidence that they are serially homologous 

 seems convincing. 



Normally there is a gap of a few segments between the hind end 

 of the pronephros (segt. 15) and the front end of the opisthonephros 

 (segt. 24) but Brauer found that even in these segments there makes 

 its appearance a distinct nephrotome, with the vestige of a glomerulus, 

 although it does not proceed with its development. Consequently 

 the units of pronephros and opisthonephros (primary) are to be 

 regarded as members of a once continuous series. That this series 

 once extended back beyond the present limits of the opisthonephros 

 is indicated by the fact that distinct nephrotomes are present in 

 segments 101-104, but as was the case in the intermediate zone 

 between pronephros and opisthonephros these do not proceed to 

 develop tubules. 



Elasmobranchii. — It will be convenient now to consider shortly 

 the development of the opisthonephros in the Elasmobranch fishes as 

 they have provided the material for a large proportion of the most 



