Normal Plates of the Development of Lepidosiren paradoxa and Protopterus annectens. g 



must remain somewhat doubtful, the general course of development of the pronephros is fairly clear. About 

 stage 21 the coelomic cavity makes its appearance in the" nephrotome and rapidly extends out into the 

 hitherto solid tubule rudiment, and later on back into the archinephric duct. Of the series of nephrotomes 

 which go to compose the pronephric rudiment on each side only two normally proceed with their development 

 to form functional pronephric tubules, the fully developed pronephros being characterized like that of 

 Ceratodus, Polypterus and many Urodeles by its having two functional tubules. In both Lepidosiren and 

 Protopterus the pronephric tubules which become functional appear to be those corresponding with myotomes II 

 and IV. As an occasional Variation three tubules may be developed (II, III, and IV). In those nephrotomes 

 which develop functional tubules the nephrotome itself goes on developing, its cavity becoming widely 

 patent to form a pronephric Chamber. In enibryos with three tubules there is seen to be a corresonding 

 series of three pronephric Chambers lying one behind the other and at first without any open communication 

 between successive Chambers. Later on the pronephric Chambers on each side become confluent so that 

 there is now a single Chamber stretching through the region occupied by myotomes II, III and IV. The 

 floor of the pronephric Chamber becomes pushed upwards about stage 24 by a blood sinus which expands 

 beneath it opposite each nephrostome to form a dorne -shaped swelling, the rudiment of the glomerulus. 

 These originally separate glomerular rudiments soon fuse so that each Compound pronephric Chamber 

 contains a single elongated glomus. By differential growth of the wall of the pronephric Chamber the 

 glomus has its point of attachment gradually shifted mediad and dorsad so that by stage 30 it Springs 

 from the dorsomedial angle of the Chamber i. e. from close to the now median dorsal aorta. As will have 

 been gathered the pronephric Chambers or Chamber are at first in perfect continuity with the splanchnocoele 

 which spreads outwards from them. Later on as the pronephros becomes greatly enlarged it bulges 

 prominently across the splanchnocoele towards the mesial plane and eventually about stage 32 it comes in 

 contact and fuses with the mesodermal sheath of the alimentary canal. In this way the glomus comes to 

 be enclosed in a secondary pronephric Chamber which however remains in free communication with the 

 splanchnocoele at its hinder end. The glomus becomes firmly slung diagonally across this Chamber by its 

 tip undergoing fusion with the mesodermal capsule of the pronephros upon the ventrolateral side of the 

 Chamber posteriorly. 



To return to the pronephros itself. From about stage 26 onwards the pronephros increases rapidly 

 in size owing to the rapid growth in length of the anterior end of the archinephric duct and also — though 

 to a much less extent — of the tubules. The anterior part of the archinephric duct increases so rapidly 

 in length that it becomes greatly convoluted. This marked increase in length of the anterior part of the 

 duct is accompanied by great dilatation of its cavity and thinning of its walls. These phenomena may be 

 correlated with the fact that the cloacal opening becomes closed about the time of hatching and, there 

 being no allantoic or other urinary reservoir posteriorly, the urinary fluid driven out through the still active 

 pronephric tubules has perforce to accumulate in the pronephric duct and causes great distension as it 

 does so. As is well known a similar effect appears to be brought about in some Amphihia by an occlusion 

 of the archinephric duct (Marshall and Bles). Rapid degeneration of the pronephros now sets in. Its 

 walls assume a waxy appearance, atrophy takes place and by about stage 36 it has practically disappeared 

 although the archinephric duct can still be traced forwards for some distance in front of the mesonephros. 



Archinephric ducts. 



The question as to how the archinephric duct extends backwards is one to which it is very 

 difficult to find a certain answer. In a perfectly preserved embryo both ectoderm and mesoderm are fitted 



Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. X. 2 



