74 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



be divided into two sets, in one of which the elements (those of the 

 "first order") are metamerically an-anged. It is suggested that the 

 units of the "second order" represent the final product of a phylogenetic 

 evolution in which a number of secondary units liave been transformed 

 into apparently primary ones. Their similarity to the true primary 

 units (those of the first order) is due to the fact that they now arise 

 almost simultaneously with these, and hence are developed under 

 identical conditions. It is further suggested that the units of the 

 second, third, etc., sets in Ichthyophis probably represent a stage in 

 the evolutionary process intermediate between typical dorsal units and 

 the extra primary ones (those of the second order) of Urodela, in that 

 they connect with the duct, rather than with the tubules of the primary 

 units, but have not as yet formed connections with the gonads. 



5. One of the results of the early nppearance of tlie extra primary 

 units (those of the sacond order) in Urodela is that, instead of the entire 

 niesomer of each somite being converted, as in Ichthyophis, into 

 a single mesonephric blastula which gives rise to later generations by 

 budding, the mesomer itself gives rise directly to more than one unit. 



G. The appearance of the fundament of the dorsal sets of units takes 

 place much earlier than has been hitherto supposed. They arise by 

 a splitting or budding process from the primary blastulae shortly after 

 tlie latter are formed, and while they are still small and simple. The 

 tertiary sets arise in a similar manner from the secondary, the quaternary 

 fiom the tertiary, etc. 



7. At the period of the animal's metamorphosis only primary and 

 secondary units have produced outer tubules, and in all jirobability the 

 outer tubules are confined to these two sets of units throughout life. 



8. By plotting the positions of the units at various ages it is learned 

 that after the primary units are clearly differentiated in a somite no more 

 are added, the apparent increase in number in certain somites being due 

 to a concentration of the whole organ. There seems to be no degeneration 

 of units in the ages examined. 



9. The plotted diagram (p. 56) also brings out the fact that the 

 appearance and development of units, caudad, is not uniform, there 

 being a delay between their completion in the sexual and an anterior 

 part of the secretor}'^ portion and their beginning in the remainder of 

 the secretory portion. This fact is considered as important additional 

 evidence in favor of the view, first suggested by Semper in 1875, that 

 the secretory portion of the mesonephros in some of the Anamnia is the 

 morphological representative of the amniote metanephros. 



