358 RUTH B. ROWLAND 
ney and of its duct — such compensation as is common in the 
adult kidney of the higher vertebrates. The degree of com- 
pensation which has been attained by the single kidney has been 
estimated in terms of increase in the secreting surface of the 
pronephric tubules, as well as by measurement of the volume of 
the cells making up the walls. 
Although the literature on the subject of hypertrophy in the 
kidney of the higher forms is too extensive to permit af a dis- 
cussion in any detail in the present paper, it maj^ be well to 
mention several of the more important standpoints from which 
the subject has been treated. 
The question of direct causal connection between the demand 
for increased functional activity and the changes in the com- 
pensating organ has been definitely settled by such experimental 
studies as those of Sacerdotti ('96), in which the kidneys of 
unoperated dogs were stimulated to compensatory overgrowth 
by injection of the blood from completely nephrectomized ani- 
mals. In this instance, as in all the early pathological literature 
dealing with this subject, the exact nature of the histological 
changes evoked in the stimulated organ was given only second- 
ary consideration. Recently, however, with the general accep- 
tance of the distinction between the terms hypertrophy and 
hyperplasia, more accurate observations of the condition of the 
kidney constituents consequent on increased activity have been 
made. Both forms of regulation may occur in the same organ, 
each being limited to a definite area. Wolff ('00), in his con- 
tributions on the macroscopic and microscopic conditions of the 
hypertrophied kidney after resection, draws a sharp distinction 
between those changes which occur in the region of the lesion, 
and those occurring in the uninjured portions of a resectioned 
kidney. In the former location he observes that mitoses appear 
at the end of two days, resulting in the formation of new epithe- 
lium. In the uninjured portion of the remaining kidney tissue, 
however, no new formation of either urinary tubules or of glo- 
meruli takes place as compensation for those excised, but here a 
sufficient restitution occurs through increase in size, and the 
normal balance is restored. From the histological viewpoint, 
