70 bulletin: museum of comparative zoology. 



Order of Apjjearance and Number' of Primary Units. 



It is usually stated that in Anura the posterior mesoiiephric units are 

 the first to appear, the more anterior ones following in order. This is 

 not strictly true for R. sylvatica. As will be seen from Diagram 2 

 (A-T), from four to nine primary fundaments, in the shape of swellings 

 in the blastema, appear simultaneously . Of these the more posterior 

 develop most rapidly, so that in passing cephalad one finds the blastulae 

 less and less mature. 



In none of the larvae nineteen millimetres or less in length whose 

 mesonephric imits are plotted in Diagram 2 (A-T) are there more 

 than nine primary units. In larva T there are twelve collecting trunks 

 opening into the duct ; in larva U there are eleven, and in one indi- 

 vidual (32 mm. long), not represented in the diagram, there are fifteen. 

 There are two possible explanations of the preponderance in the number 

 of these collecting trunks over that of primary units in the younger 

 specimens. Either new primary units have been added, or some belong- 

 ing to tlie secondary set have sent their tubules directly to the Wolffian 

 duct instead of to the collecting trunks of the primary units. Tliat the 

 number of tubules opening into the duct in larva T exceeds by three 

 the number of units plotted as primary ones, seems to favor the latter 

 view, but it must be borne in mind that the units were plotted as be- 

 longing to the primary or secondary set solely from the position of the 

 Malpighian body, and that this may easily have been displaced by 

 crowding. "Whether or not secondary units have simulated primary 

 ones by sending their tubules directly to the duct, there is no doubt 

 that true primary units are added to the original series, for in both 

 larvae T and U, young, deeply staining ones are seen at botli the poste- 

 rior and anterior ends of the kidney. Even in the older larva (32 mm. 

 long) mentioned above, immature primary tubules are seen at the poste- 

 rior end of the series. In this larva the most anterior tubule on eitlior 

 side of the body, although it is clearly functional, is not accompanied l)y 

 any sign of a fundament of dorsal sets. From this it se?nis probable 

 that at least one tubule may remain simple throughout life. 



Diagram 2 (p. 65) shows that there is some shortening of the raeso- 

 nephros. Much more sti'iking is the shortening of the germ-cell mass 

 (principally the posterior portion), which either shrinks by a rearrange- 

 ment of cells or undergoes degeneration. The same phenomenon was 

 noted in Amblystoma. 



