66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



of the first order," one for each somite, appeared in the posterior part of 

 the mesomers before these were cut olf from the epimers to form the 

 mesonephric blastema. The question naturally arises as to whether 

 there is in Rana any such sign of the primitive metameric arrangement 

 of the mesonepliric units. I find no such condition as that described in 

 Amblystoma. The only hint at a segmental arrangement is found in a 

 few cases (Diagram 2, larvae B, G, K, P), where there is but one " swell- 

 ing " per somite. As suggested above, some of these swellings may really 

 contain the fundaments of more than one blastula. This is difficult to 

 believe, however, in such cases as larvae A" and P, for there the swellings 

 are in the shape of distinct blastulae (Figure 15 shows the one in the 

 twelfth somite of larva A"), and it seems probable that, w^ere any of them 

 going to divide, some signs of the fact would already be observable. It 

 seems possible, then, that such cases show an atavistic condition, and that 

 if a large number of adults were examined, cases might be found showing 

 the primitive condition of a strictly segmental arrangement of iho primary 

 units of the mesonephros. 



Development of Primary, and Origin of Secondary, Blastulae. 



As development proceeds, the blastulae grow more massive and re- 

 semble those of Amblystoma, with the exception already mentioned that 

 their anterior and posterior ends are prolonged to form a connecting cord. 

 In later stages, this cord is often clearly interrupted, generally nearer the 

 posterior than the anterior blastula. Even when the cord seems to be 

 continuous, it is possible that there is really only close contact between 

 successive blastulae. Figure 102 (Plate 8) shows the relative size of 

 two blastulae and connecting cord in the posterior portion of the meso- 

 nephros, — where the blastulae, being more numerous, are naturally 

 more closely approximated than farther forward. This figure was made 

 by superposing drawings of six consecutive frontal sections.-^ One of the 

 sections used in the reconstruction is shown in Figure 18, Plate 2. As 

 the blastula was cut obliquely, only its posterior prolongation shows. 



For the sake of clearness and brevity I have illustrated the differenti- 

 ation in a mesonephric unit by a series of six diagrammatic cross-sections 

 (Figs. 94-100, Plate 8). Figure 94 represents a cross-section at the stage 

 when the blastula has the form of a simple, longitudinally elongated 

 spindle. The next step in the process, represented in Figure 95, is the 

 partial division of the blastula into a smaller dorsal and a larger ventral 



1 This method gives only a silhouette and lience affords no proof of the conti- 

 nuity of the connecting rod. 



