700 Emily rat gregorY, 



ing about this, the following facts seemed to me to throw some light 

 ou the subject : 1) Even in stage XIX, while the metanephros is little 

 more than a basis, the mesonephros appears to have reached the height 

 of its development, the nephric lobe is very large, and leaves little 

 space in the body cavity, the tubules consist of large cubical cells, 

 certainly not capable of much further growth, and we see no mitotic 

 figures. 2) In stage III, the neurenteric canal is but fifteen sections 

 (150 f.i) behind the opening of the Wolffian duct into the cloaca, 

 therefore, this is the region from which the mesodermal sheet first 

 grew forward, and where, if at all, we should expect to find embryonic 

 cells. 3) The appearance of the tissue is that of embryonic material, 

 with its close-packed, densely-staining elements. 



If one may venture to speculate a little, it seems as if the 

 growth of the embryo demanded a better excretory system. The 

 mesonephros had reached its highest possible development and could 

 not grow and meet the new demand, all the embryonic tissue in the 

 anterior end of the body had been appropriated by the various or- 

 gans and there was no room in the body cavity for another kidney. 

 The point at which the new growth begins seems to be the only one 

 where the increasing need, whatever the stimulus, could be met by 

 the formation of another organ. 



The conclusions reached by means of these researches may be 

 summed up as follows: 



1. The pronephros of the turtle arises as outgrowths from the 

 posterior somatic region of the somites, having the fourth and tenth 

 as its anterior and posterior limits, respectively, and shows such 

 variation and irregularity as is to be expected in a rudimentary organ. 

 The posterior tubules more or less fused with mesonephric elements, 

 function as excretory organs. 



2. The mesonephros of the turtle may extend anteriorly over 

 much of the pronephric region and fuses with it so that the parts 

 can only be distinguished in the earliest stages. 



3. a) The metanephros has its origin where the ureter branches 

 from the upper side of the Wolffian duct and in the blastema sur- 

 rounding it. 



b) Apart from this point of union, and perhaps a few cells from 

 the mesonephric blastema the metanephros arises in entire inde- 

 pendence of the mesonephros. 



c) The tubules of the metanephros arise independently in the 



