hall: mesonephcos and mlllepjan duct in amphibia. 75 



10. The extent of the germ-cell mass, caudad, becomes less as the 

 animal grows okh-r. This is due to a total disappearance of tlie germ 

 cells, as such, they being transformed in all probability into ordinary 

 peritoneal cells. A similar transformation is suggested to account for 

 their disappearance from the somatoderm in the sexual portion of the 

 mesonephros. 



Rana. 



1. As in Amblystoma, the mesomer contains tissue from both soma- 

 toderm and splanchnoderm. In the region of the mesonephros, the 

 mesomers detach themselves from the rest of the somite and fuse to form 

 a continuous mesonephric blastema, in which swellings are seen. These 

 are the mesonephric blastulae, — the fundaments of the mesonephric 

 imits. In a few cases they were found to correspond in number to the 

 mesomers from which the blastema was derived (that is, are metameric in 

 arrangement), but usually there is more than one to each somite. The 

 distinction into swellings of the " first order " and those of tlie " second 

 order," as observed in Amblystoma, cannot be made in Rana. The 

 blastulae differ from those of Amblystoma in that they are (or seem to 

 be) usually joined to each other by their tapering ends. 



2. The dorsal sets of units develop from a fundament derived from 

 the blastula of the primary unit, essentially as iu Amblystoma. This 

 fundament, however, instead of being early cut off from the primary 

 blastula, retains its connection. "When the development of the primary 

 blastula is completed, this connection is found to be with its Bowman's 

 capsule. As in Amblystoma, there seems to be one secondary unit fur 

 each primary, one tertiary for each secondary, etc. The number of 

 dorsal units communicating with the collecting trunk of each [)riraary 

 decreases cephalad at such a rate that the most anterior primary may 

 not possess even a secondary unit. There is not, however, as in Uro- 

 dela, an extended region in which primary units are alone present. 



3. The most striking peculiarity in the development of a mesonephric 

 unit in Anura is found in the behavior of the outer tubule, — a peculiar- 

 ity already observed by several authors. Each of the earlier outer tub- 

 ules arising from the primary units grows out from the blastula in the 

 manner described for Amblystoma. Shortly after its distal end reaches 

 and coalesces with the peritoneum, its proximal end severs its connec- 

 tion with the rest of the unit and opens into a vein. Later in life many 

 additional outer tubules appear. In structure, these are similar to the 

 first ones, but their orujin is quite different, and difficult to determine. 



