226 EMBBYOLOGY OF THE LOWEK VEKTEBEATES ch. 



Additional tubule rudiments to the number of about 8 arise in 

 order behind those first laid down. They arise in exactly the same 

 way as the first (Fig. 122, B, VII) but as the archinephric duct 

 rudiment has already grown past their point of origin they become 

 joined on to the duct by their outer ends undergoing secondary 

 fusion with it. Each tubule rudiment grows actively in length so , 

 that it eventually forms a much-coiled tube connecting the nephro- 

 coele or coelomic cavity of the nephrotome with that of the duct. 



In the meantime the nephrotome is undergoing important 

 changes apart from the ' tubule rudiment. Its cavity, the nephro- 

 coele, from being a mere slit with its floor and roof in contact, 

 becomes widely dilated and it becomes cut off from the dorsal part 

 of the segment which forms the myotome and sclerotome (Fig. 123, C). 

 The nephrotome also becomes gradually constricted off from the 

 lateral mesoderm but in this case the separation either never 

 becomes completed (Fig. 123, C, p.c) or if it does so, is merely 

 temporary — communication being soon re-established at the point 

 where the constriction took place. The nephrocoele is thus, even 

 in the fully developed pronephros, in open communication with the 

 splanchnocoele by a more or less narrow channel the peritoneal 

 canal (Fig. 123, C, p.c), the splanchnocoelic end of which forms the 

 peritoneal funnel. 



As the nephrotome and tubule go on with their development 

 there arise characteristic relations with the blood-vascular system. 

 An intersegmental branch from the dorsal aorta passes to each 

 nephrotome, causing its floor to bulge into the nephrocoele (Fig. 

 123, C, gl) and form a conspicuous projection — the glomerulus — 

 which later on fills up most of the nephrocoelic space. From the 

 glomerulus the vessel passes (as the vas efferens) into a network of 

 blood-spaces lying between the coils of the tubule and belonging to 

 the posterior cardinal vein. 



The fully formed pronephros of HypogeopKis is composed of 

 about a dozen segmentally arranged units each developed in the 

 way described. It is to be noted however, that the last three of 

 these units never become fully developed and further that behind 

 the last as well as in front of the first unit of the functional 

 pronephros each segment has its typical nephrotome though this 

 never proceeds with its development. In other words the pro- 

 nephros of Rypogeophis possesses at its anterior and posterior ends 

 a number of units more or less reduced or vestigial. 



It is also of interest to notice certain variations which occur in 

 connexion with the relations of the tubule to the nephrotome. In 

 what may be termed the typical arrangement the nephrostome 

 opens from the dilated part of the nephrocoele (Fig. 124, A). 

 Frequently however it has become shifted on to the constricted peri- 

 toneal canal (Fig. 124, B). When it does this there are apt to arise very 

 misleading appearances as illustrated by the accompanying figure, 

 whereby on the one hand the tubule appears to lead directly from 



