iv OPISTHONEPHKOS 253 



the secondary. As the various generations of tubules go on with 

 their development, undergoing the same histological differentiation 

 and increasing enormously in length, they become inextricably mixed 

 up together to form the compact fully developed opisthonephros 

 of the adult. 



Eventually, in the Urodele, the duct is slightly displaced out- 

 wards so as to leave a distinct gap between it and the opisthonephros 

 across which pass the terminal parts of the collecting-tubes. In the 

 male Urodele the openings of these become, as a rule, shifted back- 

 wards to the hind end of the duct as in Elasmobranchs. 



The Amphibia alone among tetrapod Vertebrates retain the 

 relatively primitive feature of possessing open peritoneal funnels in 

 the adult, and they can be excellently demonstrated with their 

 actively moving flagella by examining the slender anterior portion 

 of the excised and still living kidney of a female Urodele in normal 

 salt solution under the microscope. In the anterior genital portion 

 of the opisthonephros of the male they as a general rule (not in 

 Spelerpes, Spengel) remain however obliterated. 



In the Anura (Nussbaum, 1886) the peritoneal canals at an early 

 stage of larval life lose their connexion with the Malpighian body or 

 tubule and establish a secondary connexion with the blood spaces 

 between the tubules, thus affording a route by which the fluid in the 

 splanchnocoele is returned to the blood, analogous to that provided 

 by the lymphatic system in higher Vertebrates. 



Amniota. — In the Amniota the opisthonephros of the Fishes and 

 Amphibians is represented by the mesonephros and metanephros — 

 and it will be convenient to consider the mesonephros first. 



Mesonephros qf Birds. — As has already been pointed out one 

 of the marked differences between Amphioxus and the Craniata is 

 that in the latter segmentation is no longer apparent at any stage of 

 development in the ventral or splanchnocoelic region of the meso- 

 derm. The Amniota show a further accentuation of this difference 

 inasmuch as the loss of mesodermal segmentation has extended so 

 far towards the dorsal side as to involve the region of the nephro- 

 tomes. In the early embryo of the bird the nephrotomic part of the 

 mesoderm has the form of an unsegmented mass — the intermediate 

 cell-mass — showing more or less distinct traces of being composed of 

 a somatic and a splanchnic layer continuous with the correspond- 

 ing layers of the splanchnocoelic mesoderm and" of the myotome. 

 Although the intermediate cell-mass no longer consists of discrete 

 nephrotomes, traces of its primitive segmental nature persist in its 

 connexions with the segmentally arranged myotomes and in the fact 

 that its connexion with the lateral mesoderm is not continuous in a 

 longitudinal direction. 



As regards the mode of origin of the actual mesonephric units 

 differences exist, as was shown long ago by Sedgwick (1880), which 

 are of much interest owing to the fact that the less modified mode of 

 development found at the. front end of the series is readily correlated 



