REMOVAL OF PRONEPHROS OF AMBLYSTOMA 377 
there being 1950 nuclei in PN 7 d and 2252 in PN 7. Although 
this may indicate a certain amount of hyperplasia, that is, 
increase in actual number of cells present, it is not large enough 
to account for the great percentage of increase in both secreting 
surface and in volume. This increase must be mainly attrib- 
uted to actual hypertrophy or enlargement of the cells already 
present. 
Effect of unilateral excision on the glomerulus 
A study of the condition of the glomeruli subsequent to double 
excision was necessarily limited by the early death of the embryos. 
The occurrence of the glomerulus in these cases has already been 
noted. On examining larvae from which only one pronephros 
had been removed, both glomeruli were found to be invariably 
present. The one on the operated side, however, exhibited less 
uniformity in size and shape than the normally functioning one. 
In some cases the outer layer of the glomerulus and the epithelial 
lining of the body wall had coalesced, this being the case not 
only on the operated, but on the normal side as well. On the 
operated size, however, the distance to be bridged is very greatly 
increased, since the absence of the pronephric swelling increases 
the width of the coelomic cavity there (fig. 13). 
Since the development of this part of the kidney unit is quite 
independent of the presence of the tubular portion, its functional 
activity can be counted on to continue undisturbed. No hyper- 
trophy in glomerular structure, then, would be anticipated, nor 
was such found to be the case, for the structural conditions 
existing allow free passage of the filtrate ventrally from one side 
to the other through the coelomic cavity. Filtration continues 
on both sides, the demand for increased physiological activity 
falling only on the tubules of the unoperated side. 
Effect of unilateral excision on other components of this system 
Removal of one head kidney has a widely varied effect on the 
formation of the segmental duct on the operated side. The 
process of development of the non-functioning ducts took place 
