90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



permanent character. It is true that it becomes well established only 

 during the degeneration of the pronephros, but this fact is of little sig- 

 nificance, from a phylogenetic standpoint, to those who would homologize 

 the pronephric glomerular cavity with those of the mesonephros. 



B. Ran A. 



As is well known, the three nephrostomes in Anura open separately 

 into a portion of the body cavity which is partially cut off from the 

 general body cavity by a fusion of the lung with the lateral body-wall, 

 that is, with the surface of the pronephi'os. Tliis pocket contains the 

 glomus and is called the glomerular cavity. In Rana sylvatica, imme- 

 diately before the degeneration of the pronephros, the glomerular cavity 

 is forced to a more nearly median position and much reduced in size by 

 the growth of a shelf ^ (Plate 5, Fig. 65, tab.) comparable with that 

 found in Amblystoma. As the pronephros retains its original position, 

 the nephrostomal tubules are compelled to elongate in order to retain 

 their connection with the glomerular cavity. The three nephrostomal 

 tubules, which originally opened quite far apart, converge toward a 

 common point, so that they finally acquire a common opening, which I 

 shall term the nephrostomal vestibule, or common nephrostome. Figure 

 65 shows this common nephrostome {vst. nph.), and also the second 

 nephrostomal tubule (thl. pr^nph.) one section anterior to its opening 

 into the common nephrostome. This condition, which, to ray knowl- 

 edge, has never been mentioned in descriptions of the degeneration of 

 the pronephros, may, of course, exist only in R. sylvatica. In that form 

 it exists, almost without exception, in all the larvae over twenty-five 

 millimetres in length which I have examined. The common nephro- 

 stome bears some resemblance to, and may be homologous with, the 

 nephrostomal cavity in Amblystoma. Just as the first nephrostome in 

 Amblystoma was finally cut off by the closure of the nephrostomal 

 cavit}^, so here the three nephrostomes become cut off from the coelom 

 b}'' the closure of the common nephrostome. 



To anticipate a little, the later history of this common nephrostome is 

 illustrated by Figures 68-77 (Plates 6, 7). It gradually becomes deeper 

 and narrower until it resembles the ordinary nephrostomes. Like them 

 it is ciliated, and may have been mistaken for the third nephrostome, 



1 The shelf in Rana sylvatica extends caudad but a short distance and is a 

 temporary structure. Very often the lung, which early loses its connection with 

 the lateral body-wall, is fused with the ventral side of the shelf. 



