68 bulletin: museum of comparative zoology. 



The succeeding dia,qrammatic figures (98. 99, and 100) scarcely need 

 explanation. Tlie only differences between the condition shown in Fig- 

 ure 99 and that seen in actual sections are : (1) the outer tubule is not 

 encountered in the same section as the inner tubule, because it enters 

 the Malpighian neck from a slightly anterior direction ; and (2) the 

 fundament of the dorsal sets of tubules {fnd. ms'nph. 2) is also dis- 

 placed, being generally posterior, though sometimes anterior, to the rest 

 of the unit, and hence is not seen in a section through the centre of the 

 blastula. Although on account of this displacement the relationship 

 between the dorsal fundament and its parent cannot be shown in a single 

 section in the case of the primary unit, it can be shown in the case of 

 the secondary, where often there is no displacement. Thus Figure 29 

 (Plate 3) shows the secondary blastula and the fundament of the more 

 dorsal sets {fnd. ms^nph. 3), connected by an attenuated cord. A cor- 

 responding stage of the primary unit is represented in Figure 100. 



To recapitulate briefly, the fundament of the dorsal sets of units is 

 developed from a *' dorsal chamber " constricted off from the primary 

 blastula. This dorsal chamber, at first tubular, becomes a typical 

 blastula, probably by a shrinking or shortening of the tube to form a 

 spherical mass. With the growth of the mesonephric mass, the dorsal 

 fundament is forced farther from the parent one and is finally severed 

 from it. For some time, however, the two remain connected by a cord 

 of cells, which indicates clearly the point of origin of the dorsal funda- 

 ment, — the dorso-median region of Bowman's capsule. 



In comparing the history of the fundaments of the dorsal sets of 

 tubules in Rana with that in Amblystoma, we see that the only essen- 

 tial difference is that in Kana each fundament remains connected for a 

 considerable period with the primary unit from which it took its origin, 

 instead of being entirely cut off from it at an early period, as in Amblys- 

 stoma. ' 



The development of the tertiary from the secondary, the quaternary 

 from the tertiary, etc., shows the same difference in the two animals. 



In the later development of the dorsal units, the opening of each into 

 the inner tubule of the next ventral (parent) one, is established in a 

 manner similar to that traced in Amblystoma, and as far as could be 

 determined, there is one secondary for each primary (throughout the 



1 The secondary units in the earliest stagres seen by Semon ('92) in Ichthyo- 

 phis consisted of spherical masses connected with Bownaan's capsule by a cord 

 of cells. From this he suggests that probably they had recently budded out from 

 the Malpighian body. 



