234 EMBRYOLOGY OF THE LOWEE VERTEBRATES ch. 



The tubule rudiments appear to arise in normal fashion, as out- 

 growths of the lateral wall of the nephrotome. These outgrowths 

 show the familiar variation of being sometimes hollow sometimes 

 solid. Thus in Amia according to Felix (1904) the anterior three 

 rudiments are hollow pockets while those farther back are at first 

 solid. 



Tubule rudiments make their appearance from segment III to 

 segment XIII but here as elsewhere only relatively few of these 

 complete their development and are to be found in the pronephros 

 at the height of its functional activity. Thus in a six-day Acipenser 

 larva Jungersen found six functional tubules while in Amia Felix 

 finds only a single tubule functional. In the latter case the tubule 

 opens' 1 from a large pronephric chamber apparently formed by the 

 fusion of at least three nephrocoeles. The tubule belongs originally 

 to the most anterior of these and corresponding to it there is present 

 a single open peritoneal canal. Later on this becomes replaced 

 functionally by another peritoneal canal situated farther' back. In 

 Lepidosteus the functional pronephros has at least three tubules 

 each with its nephrocoele (Felix, 1904). 



As in the case of the Lung-fishes the dorsal part of the splanchno- 

 coele in the pronephric region becomes floored in by the approxi- 

 mation of the mesial surface of the pronephros to the lateral surface 

 of the oesophagus (cf. Fig. 129a, E) so as to form a secondary pro- 

 nephric chamber. In Lepidosteus this forms a widely patent cavity 

 with which the first nephrocoele becomes completely merged and 

 which remains ventrally in continuity with the main splanchno- 

 coele by a narrow richly ciliated tubular channel. In Acipenser the 

 first nephrocoele undergoes a similar modification while the remain- 

 ing five are fused with one another but isolated from the splanchno- 

 coele. 



Tblbostei. — The development of the renal organs has been 

 worked out in detail in the case of the genus Salmo by Felix 

 (1897). In this case the myotomes are already separate from the 

 more ventrally situated portions of the mesoderm at a very early 

 stage. The first rudiments of the pronephros are in the form of a 

 series of somewhat conical, segmentally arranged, solid projections 

 from the median edge of the lateral mesoderm towards the mesial 

 plane. These projections — five in number (segments 3-7) in a 

 26-day Trout — are probably to be regarded as nephrotomes which 

 have been precociously separated from the myotomes, if indeed 

 they ever were continuous. These five nephrotomes soon come into 

 intimate contact so as to be no longer distinguishable. They now 

 together form a continuous mass of mesoderm the so-called pro- 

 nephric fold. The dorsal and outer portion of this mass becomes 

 nipped off to form the anterior portion of the archinephric duct 

 except at one point where a connecting isthmus remains to form a 

 tubule. The mesial portion of the mass becomes the wall of the 

 single pronephric chamber. 



