260 EMBBYOLOGY OF THE LOWEE VEETEBEATES ch. 



cloaca. This postcloacal portion drains into a pocket-like prolonga- 

 tion which grows back from the duct, usually on the right side only. 



A remarkable peculiarity has been observed in certain Teleosts 

 (Lepadogaster, Guitel, 1901) in which, correlated with the persist- 

 ence of the enlarged glomerulus of the pronephros, the Malpighian 

 bodies of the opisthonephros have, at least in the adult, completely 

 disappeared. 



Dipnoi. — In the Lung-fishes the development of the opistho- 

 nephros closely resembles that in Amphibia. In Lepidosiren and 

 Protopterus the units appear as rounded, at first solid, masses inde- 

 pendent alike of myotome and of peritoneal epithelium. In Protop- 

 terus they commence about segment 14-18 but in some specimens 

 they appear to be represented by slight condensations of mesenchyme 

 right forwards as far as the hind limit of the pronephros. The rudi- 

 ments extend back to about segment 36 i.e. to about one segment in 

 front of the cloaca. They are roughly segmental in position and 

 remain so during the greater part of larval life. Each rudiment 

 gives rise to a typical Malpighian body and a tubule which joins on 

 to the duet secondarily. There does not appear to be any trace of 

 peritoneal canals developed although they are for a time present in 

 Geratodus. 



The development of the primary units is followed by the develop- 

 ment of subsequent ones but the origin of these has not so far been 

 worked out. 



In Protopterus, though not in Lepidosiren, the hinder ends of the 

 kidneys become continuous across the mesial plane and this fused 

 portion becomes gradually marked off conspicuously by its pale 

 colour the cortical region of the paired kidneys becoming crowded 

 with amoebocytes laden with melanin which settle down there and 

 give it a coal-black appearance. 



Beptilia. — In the Eeptiles we find, as we should expect, that the 

 process of development follows upon the whole the same lines as in 

 Birds but at the same time shows various features in which the con- 

 dition remains more primitive. Thus in Lacerta Schreiner (1902) 

 finds that, except in the hinder portion of the opisthonephros, the 

 units arise directly from typical nephrotomes or protovertebral 

 stalks. These become isolated from the peritoneal mesoderm and 

 then from the myotome. Each develops a lumen and becomes 

 vesicular and its lateral wall gives rise to an outgrowth which 

 becomes the tubule rudiment and fuses with the duct. No peri- 

 toneal canals are developed, though vestiges of these may appear — a 

 vestigial peritoneal funnel appearing as a slight projection from the 

 splanchnocoele into the ventral end of the nephrotome (Lacerta), or 

 the latter remaining for a time connected with the peritoneal lining 

 by a solid stalk representing the peritoneal canal (Anguis). 



In the posterior segments the nephrotomes are no longer 

 distinct: they form a continuous mass of mesenchyme stretching 

 uninterruptedly from segment to segment. In this, cellular conden- 



