HALL: .MESONEPHROS AND MULLEKIAN DUCT IN AMPHIBIA. 61 



and Amblystoma. They resemble the primary in their position and in 

 the fact thai they open into the duct ; they resemble the true secondary 

 units in the fact tliat their outer tubules are formed after the unit 

 has become quite complex, and in the fact that they do not form 

 connections with the testis. Semon himself suggests that in Urodela 

 the connecting of all of the primary units with the testis is a secondary 

 acquirement. 



Relations between Sexual and Secretory Portioyis of the Mes(mephros. 

 As is well known, the mesonephros in Urodela is divided into a sexual 

 and a secretory portion. The former is connected with the sexual glands 

 by outgrowths from Bowman's capsules, permanently in the male, tem- 

 porarily in the female. In the male the outer funnels of this portion of 

 the kidney close and disappear. The secretory part, which lies posterior 

 to the sexual gland and does not form connections with it, is character- 

 ized by the presence of dorsal sets of units. I will next consider the 

 extent of tljese two parts in Amblystoma. From Diagram 1 it will be 

 seen that scattered germ-cell masses may extend as far back as somite 

 19 (larva G). In the older larvae (from 23 mm. in length), however, 

 the germ cells become restricted without exception to somites anterior 

 to the sixteenth. Thus the sexual part of the mesonephros cannot 

 extend back of the anterior end of somite 16. The rest of the kidney 

 is to be considered as belonging to the secretory part. It will be noticed 

 that a criterion which is generally used in defining the secretory part, 

 namely, the presence of dorsal units, does not apply to the whole of 

 the part, for those units do not extend farther cephalad than the 

 posterior end of somite 17. Although they may be added to somites 17 

 and 16 at a later date, it seems very improbable that they are, from 

 the fact that the formation and development of those already present 

 progressed from in front backward. The posterior portion of the secre- 

 tory pai-t is thus distinguished from the anterior portion by an anatomi- 

 cal character, — tlie presence of dorsal units. Significance is given to 

 this difference in anatomical character by a peculiarity in the develop- 

 ment of this region of the mesonephros which, although not conspicuous, 

 is to be considered as of some importance from a phylogenetic stand- 

 point. The peculiarity consists in a retardation in the apjjearance and 

 development of the hlastulae. When the blastulae first become distinct, 

 they extend back to the region of somite 16 (see Diagram 1, larvae C, 

 D, etc.). From that stage to one represented by larva /, — a period of 

 rapid development during which the animals have added six millimetres 



VOL. XLV. — NO. 2 3 



