HALL: MESOXEPHROS AND mQlLERIAN DUCT IN AMrillBIA. 91 



^yllich is generally described as persisting to a late stage. In Figure G8 

 is seen the end of the first nephrostomal tubule, in Figure 69 its open- 

 ing into the common nephrostome. The connection of the latter with 

 the coelom is shown in Figures 70, 71, and 72. The second nephrosto- 

 mal tubule is shown in the same three figures, and the third in Fig- 

 ures 75-77. The first nephrostomal tubule comes into the common 

 nephrostome from an anterior direction ; the second from a point directly 

 laterad, and the third curves dorsad and then cephalad. 



The first sign of the fundament of the MiiUerian duct appears in the 

 form of a thickening of the peritoneal epithelium of the pronephros at a 

 little later stage than the one from which Figure 65 was taken. It wuU 

 be remembered that the peritoneal evaginations in Amblystoma which 

 formed such an important part of the fundament of the MUllerian duct, 

 arose just ventral to the two nephrostomes. The conclusion is obvious 

 that the nephrostomes may in some way determine the position and 

 number of these evaginations. If that is true, we should expect to find 

 three evaginations in Rana, one beneath each nephrostome. But the 

 fundament of the MiiUerian duct does not arise until after the migration 

 of the three nephrostomes to a common region, so that the tissue imme- 

 diately ventral to them is now probably represented by tissue just 

 ventral to the common nephrostome. Bearing this in mind, as ^vell as 

 the fact that the embryological processes in the highly specialized Anura 

 are often hurried and obscure, as compared with those in the Urodela, I 

 was not surprised to find that the theoretical three evaginations are 

 generally represented by a single irregular mass of cells proliferated from 

 the peritoneal epithelium. This mass is always elongated antero-poste- 

 riorly, and lies ventral to the common nephrostome. It usually shows 

 more than one obscure evagination from the coelom extending into it. 

 Only once did I find it represented by one large, well-formed evagina- 

 tion. I searched carefully for evidence that the three evaginations are 

 not purely theoretical, and was fortunate enough to find one case in 

 which the complete fusion of the three nephrostomes had been retarded 

 until after the appearance of the fundament of the MiiUerian duct. In 

 this larva, on one side, the two anterior nephrostomes are fused, but the 

 third still opens separately at some distance behind the common open- 

 ing of the anterior ones. Ventral to the opening common to the first 

 and second nephrostomes, there is a distinct evagination of the peri- 

 toneum (Plate 5, Fig. 66). From this a thickened band runs ventrad 

 and cephalad over the face of the pronephros along its line of junction 

 wdth the shelf. Ventral to the third nephrostome is a thickened dish 



