252 BULLETIN OF THE 



Stage IV. 



Lai'vse of Amblystoma do not possess a conspicuous widely open 

 pronephric pouch, such as has been described in Anuran species; but 

 the proliferation becomes at once converted into a tubular organ. In- 

 deed, the condition of the pronepliric thickening in Stage III. is the one 

 which is most similar to the Anuran pronephric pouch, since it is then 

 a continuous structure having connections with the coelom in segmental 

 regions. 



In slightly older embryos, the dorsal half of the pronephric thicken- 

 ing is no longer continuous through the region between protovertebrse 

 III. and IV. ; and from this region backward to the hinder face of pro- 

 tovertebra IV. the mass is distinctly divided into two tubes. Of these 

 two tubes, the more median and dorsal is the second nephrostomal 

 tubule ; the more lateral and ventral is the common trunk. Finally, 

 it is to be observed in a number of cases that an anterior loop of the 

 common trunk occurs a short distance in front of the point of junction 

 with the nephrostomal canals. The pronephros thus has a form which 

 approximates very closely to the condition which forms the starting 

 point for the next stage (Plate VIII. Fig. 58). 



Stage Y. 



Plate VIII. Figs. 57-60. 



This stage includes embryos which have attained a length of from 

 5 to 6 mm. Many of the older embryos of the stage have already 

 hatched ; they possess well developed tails and swim about freely. 



The general form of the pronephros has been studied by means of a 

 number of rough reconstructions, some of which are represented by the 

 diagrams on Plate VIII. In Figures 57 to CO inclusive, which belong 

 to this stage, no windings have been reproduced which were not of suf- 

 ficient magnitude to form definite antero-posterior loops ; and, further, 

 in plotting these loops, no attempt has been made to preserve in the 

 diagram the natural direction in which the tube is actually bent. How- 

 ever, the relative positions of the bends in an antero-posterior direction 

 have been accurately reproduced. 



In the younger individuals of this stage, the pronephros (Fig. 58) 

 resembles in many respects that of Rana represented in Figure 33 ; but 

 it differs from the latter, notably in the occurrence of two instead of 

 three nephrostomes and nephrostomal canals. For this reason, there is 

 no canal which corresponds to the collecting trunk of Anura, save that 



