284 BULLETIN OF THE 



Urodela. At first sight it would seem probable, — and by implication I 

 accredit this opinion to Mollier, — that the rudimentary third tubule 

 occasionally present in Urodeles corresponds to the normal third tubule 

 of Anura. This view, however, is not in precise harmony with the rela- 

 tions of the nephrostomes to the myotomes. As I have already shown, 

 the first nephrostome in Amblystoma is situated beneath myotome III., 

 wliereas in Rana and Bufo it occius under myotome II. If now the 

 enumeration of the somites in the two cases correspond, it follows that 

 the first and second nephrostomal tubules of Amblystoma are equivalent 

 to the second and third tubules respectively of Rana and Bufo, not to 

 their first and second tubules, and that the occasional rudimentary third 

 tubule of Urodeles belongs to a more posterior somite, and is unrepre- 

 sented in Anura. In Amblystoma the raot of the vagus nerve arises 

 immediately in front of the somite which I have denominated I. ; ^ the 

 same is true in the case of Rana and Bufo, and I am inclined to regard 

 these as equivalent somites. It is possible that somite II. of Ambly- 

 stoma is not represented in Rana and Bufo ; but this is hardly probable, 

 since it belongs to the head region, which is hardly likely to vary in such 

 closely related groups, and since it is evident that the greater number of 

 protovertebrse present in Urodeles as compared with Anura is largely 

 accounted for by additional pro to vertebras in posterior regions, particularly 

 in the region of the mesonephros, as I believe. In general, it seems 

 to me that we should be more ready to admit the abortion of the most 

 anterior tubule in Urodela than to assert the existence of an additional 

 protovertebra in Anura. 



All the more recent writers are agreed that in Anura three pairs of 

 pronephric nephrostomes occur, Giles ('88, p. 135) alone claiming that a 

 degenerating pronephros may have four. In Urodela the typical num- 

 ber is two; but Mollier ('90, p. 224) has recorded the occasional occur- 

 rence of three pairs in Triton, and I have made similar observations in 

 Amblystoma. Spengel ('76, p. 19, Taf. II. Fig. 21) maintained, on the 

 evidence of a specimen in which the pronephros was largely degenerated, 

 that four pairs occur in Ccecilia ; the recent observations of Semon ('90, 

 p. 462), on the other hand, have shown that there exist in Ichthyophis 

 on each side of the body ten pairs of nephrostomes, thei'efore forty in all. 



1 Somite I. of this enumeration probably corresponds to the one which has 

 been called somite XI by Houssay ('91). Houssay believes that he can identify 

 in Amphibia the somites which have been observed in the head region of Selachii. 

 If his conclusions are accurate, they are evidence in favor of the view that this 

 region of the body is very permanent. 



