596 The American Naturalist. [July, 
the author’s own research. Additional evidence is presented in the 
next chapter which considers the abnormal embryos made in salt solu- 
tions in the experiments of Hertwig and of Morgan. 
Two chapters discuss the effects of gravitation acting upon cleaving 
frog’s eggs; first in the series of experiments of Pflüger and then in 
those of Born and of Roux. The tenth takes up the cleavage of eggs 
under pressure and concludes that in early cleavage the nuclei are all 
equivalent. The next throws additional light upon the nature of cleav- 
age in presenting the work of Roux, Morgan and others, who have 
reared larve from eggs in which one of the first two cells has been in- 
jured. The intricacies of “ post generation ” are also set forth here. 
The interpretations and conclusions of the twelfth chapter conclude 
that part of the volume dealing with experimental work, save that a 
short sixteenth chapter briefly gives the effect upon frog’s eggs of liem 
and of temperature. 
Two added chapters continue the account of the frog’s growth, and . 
describe the formation of mesodermal and of entodermal organs, so that 
the book may serve as a substitute for Marshall’s text-book, besides be- 
ing an introduction to a new method of study. 
In the twelfth chapter we find a very clear statement of Roux’s 
mosaic theory and its modifications, a brief account of the conceptions 
of Driesch and of Hertwig, and the remarkable facts observed upon the 
Ctenophore egg. From experiments upon these eggs the author con- 
cludes that “in the protoplasm and not in the nucleus lies the differ- 
entiating power of the early stages of development.” 
The Roux-Weismann hypothesis of qualitative nuclear division is 
rejected as having “no known histological facts in its favor.” 
To reconcile the prevailing conception of the power of the nucleus 
with the idea that the differentiating power resides in the egg proto- 
plasm outside the nucleus, the author suggests that the nucleus partici- 
pates in determining form only so far as it controls the special charac- 
ter of an organ, while the main determination as to what part shall be 
one organ rather than another, is made by the non-nuclear part of the 
egg or cell. “After cleavage, the cytoplasm of each part differentiates 
into this or that organ, but the kind of differentiation of each part is 
determined by the nucleus of that part.” 
Touching the fundamental question as to the cause of differentiation 
the author says: “ Driesch has pointed out that the egg seems to act 
like an intelligent being.” If so, are the causes of differentiation and 
of regeneration the same in kind as physico-chemical causes, or do they 
belong to the category of intelligent acts, and can these latter be ac- 
