62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



to their length, — there is no further caudad extension of blastulae, the 

 developmental processes being restricted to a maturation of the units 

 already formed. In larvae about 16 mm. in length, this maturation 

 process has resulted in the conversion of the three or four most anterior 

 blastulae into functional units. As soon as tliis occurs, the addition of 

 blastulae is resumed and progresses steadily through somites 17, 18, 

 etc., so that in a larva of 24 mm. (X) all or nearly all of the primary 

 units have appeared. During this process, this delayed set of primary 

 tubules seems to remain quiescent until all have appeared. This fact 

 explains the sudden transition, in the region of the sixteenth or seven- 

 teenth somite (larvae K, L, M), from complex units to simple blastulae 

 as one follows the organ caudad. 



Since Semper ('75) first suggested it, various authors have seen in the 

 secretory portion of the Amphibian mesonephros a forerunner of the 

 metanephros of the Amniota. Felix ('97) believes that it contains both 

 mesonephric and metanephric elements, and ]^ussbaum ('97) takes a 

 similar view. It seems to me that the phenomenon just recorded — the 

 retarded development of a part of the mesonephros — is important evidence 

 in favor of the view that a portion of the secretory part (that posterior to 

 somite 16) is comparable with the metanephros of higher forms, for one 

 of the chief peculiarities of an excretory system which is divisible into 

 meso- and meta-nephros is a chronological break iu its development. 



B. Eana. 



The earlier processes in the development of the somites in Rana are 

 very similar to those in Amblystoma. The chief difference lies iu the 

 fact that development is more hurried and the cavity of the epicoelom 

 becomes sooner filled by the ingrowth of the muscular tissue developed 

 from the splanchnic wall. The sclerotome seems to arise essentially 

 as in Amblystoma, from the splanchnoderm of the ventral wall of the 

 somite. I apply to Rana the same conception as to the extent of the 

 mesomer as in the case of Amblystoma. 



The first figure (Fig. 6, Plate 1) is of a section through the middle of 

 the sixth somite ^ of a larva 3.25 mm. in length, which corresponds to the 

 stage of Amblystoma illustrated by Figure 13, Plate 2. Although the 

 three pronephric nephrostomes are open, the Wolffian duct has not yet 



1 In numbering the somites, the three pronephric nephrostomes are considered 

 as opening in the second, third, and fourth somites. 



