60 bulletin: museum of comparative zoology. 



set, for he finds them connected with the Malpighian capsule of the 

 latter by a cord of cells which soon disappears. He further suggests 

 that a similar division of a [theoretical] first set takes place in the higher 

 Amphibia. From my description of the formation of the blastema (see 

 Stage V) it would seem that Semon's surmise as to the origin of the 

 dysmetameric arrangement of the units in the higher Amphibia contains 

 the essence of the truth. A more exact comparison between the process 

 in Ichthyophis and that in Amblystoma seems to me to be as follows : 

 in Ichthyophis that portion of tlie raesomer of each somite which 

 remains after the detachment of the sclerotome becomes entirely con- 

 verted into a single, segmental, mesonephric blastula. Any later units 

 must therefore be derived from this first one by splitting or budding. 

 In Amblystoma, the division into several units per somite takes place so 

 early that the mesomer itself is directly differentiated into the full com- 

 plement of primary blastulae belonging to its somite. Of these primary 

 units, one (that of the " first order ") in each somite appears earlier than 

 the rest and is probably homologous with the " primary " units of Ich- 

 thyophis. The later primary units (those of the "second order") in 

 Amblystoma, which soon appear between the earlier ones, are probably 

 homologous with the second, third, etc., sets of Ichthyophis. 



In Ichthyophis, the development of the second, third, etc., sets of 

 units is followed, after a long period, by the appearance of two dorsal 

 sets, whose origin is unknown. There is some evidence that these open 

 into the tubules of the earlier units, but that point was left by Semon 

 unsettled. It seems probable that these " dorsal sets" of units in Ich- 

 thyophis are alone strictly homologous with the secondary, tertiary, etc., 

 sets of Amblystoma. 



There is little doubt in my mind that the second, third, etc., sets of 

 units in Ichthyophis represent a stage in phylogenetic development 

 between tlie typical secondary (dorsal) units and the extra primary ones 

 (those of the second order) of Amblystoma. During this evolution, the 

 buds from which they arise have shifted their point of origin from the 

 dorsal side of the primary blastula to its anterior (or posterior) side. 

 This fact explains their connection with the duct instead of with the 

 collecting trunks of the primary units. The new mode of opening has 

 the gi'eat advantage of making them wholly independent of the primary 

 units, so that they are free to mature as early as the primary. The 

 theory set forth above offers an explanation of the fact that these second, 

 third, etc., sets in Ichthyophis occupy a curious intermediate position be- 

 tween the primary and the true secondary (dorsal) units of Ichthyophis 



