52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



part of the inner tubule. Viewed from the standpoint of its embryonic 

 development, I do not see how it can rightly be so considered. Func- 

 tionally, of course, it serves as an inner funnel, but, if my view of the 

 extent of the two mesomeric layers is correct, it contains tissue from 

 the splanchnic layer, — which no part of a true inner tubule does. 



The differentiation of the blastula to form a complete mesonephric 

 unitj as outlined above, will be found by comparison to agree quite 

 closely with the similar processes in Pristiurus as set forth on page 49. 



The further changes which take place in the mesonephric _^unit to fit 

 it for functioning have been too well described for Urodela to necessitate 

 a redescription. These changes consist in a widening of the various lu- 

 mina, a great increase in the length and tortuousness of the inner tubule 

 combined with a differentiation of its parts, and the establishment of a 

 blood supply to the glomerulus. 



Origin of the Dorsal Sets of Units, 



In larvae about 20 mm. in length, the blastulae which develop into 

 the primary units have become distinct as far back as the region of the 

 opening of the duct into the cloaca. Soon after these blastulae have 

 become well defined tliey are seen to be accompanied by smaller ones 

 (fnd. ms^nph. 2, Fig. 26, Plate 3), which are found from about the ante- 

 rior end of the eighteenth somite caudad. These smaller blastulae are 

 the fundaments of the secondary units. They are sometimes slightly 

 removed from the primary blastulae, but generally in close contact with 

 them. In the former case they may lie mesad to the primary ones, in 

 the latter case they are always dorsad to them. Tliere is a single secondary 

 blastula to each primary one. Although these secondary blastulae may 

 possibly develop from residual portions of the mesonephric blastema, 

 they have every appearance of arising by a division of the primary 

 blastulae.^ 



Cephalad, the secondary blastulae are generally considerably smaller 

 than the primary, whereas caudad they may nearly equal the latter in 

 size. This fact is apt to give the impression that the mode of division 

 of the primary blastulae to give rise to the secondary ones (assuming 

 that the secondary do so arise) alters as one passes caudad, but I think 

 the true explanation is, that by the time the more posterior blastulae 

 have divided, the growth of the 7:)n'wiar?/ units farther forward has out- 



1 As will be shown later, there is no possible doubt as to the origin of the 

 secondary units from the primary ones in Rana sylvatica. 



