1897.] Recent Literature. 595 
notes and papers put forth by the enthusiasm of workers in this field 
are here brought to the hands of the student in a form at once intelligi- 
ble and available; grouped about a concrete case, the development of a 
well known animal, the frog. But the book is less a compilation than 
the entire statement of the author’s own researches on this subject com- 
plimented and illuminated by facts taken from other authors, having 
a direct bearing upon the frog’s development, though sometimes going 
into wider fields for more definite illustration. It is an account of the 
development of the frog as seen from the point of view of the experi- 
mental embryologist, who desires to understand as well as to record his 
facts, and seeks information by direct, more or less pertinent, questions 
put to nature in the form of experiments. With the exception of cer- 
tain treatises upon comparative embryology, we may regard this as the 
first attempt at a truly scientific text-book of embryology—as some- 
thing more than a history of developmental stages—and it is note- 
worthy that America and not Germany has produced this as well as 
the two other important biological text-books recently issued by the 
same publishers. 
_ The book will serve as a useful manual for advanced students and 
welcome reading for the many biologists who have not the time for 
direct study in this special field. How far it may lead to the gradual 
substitution of the life history of the physiologist’s pet, the frog, in 
place of the development of the ever accessible chick in elementary 
biological instruction remains for time to show. 
A brief summary of the leading chapters may convey some idea of 
the ground covered and the treatment of the subject. The first deals 
with the making of the egg and sperm, and is largely a compilation 
with free use of phenomena better known in other animals. The sec- 
ond describes the formation of the polar bodies and the process of fer- 
tilization. The third briefly reviews experiments upon cross-fertiliza- 
tion in frogs of different species. The fourth treats of cleavage and 
shows the author’s special intimacy with this phase of the subject; 
there is not only a detailed description with excellent original figures, 
but also a most interesting comparison of the arrangements of cells in 
the cleaving of frog’s eggs with possible combinations of oil drops as 
arranged in Roux’s experiments. The author says: “ It seems highly 
probable that surface tension is also an important factor in the seg- 
menting egg, but other conditions present prevent its free play.” 
The fifth and sixth take up in detail the formation of the blastopore 
and the associated phenomena of conerescence and germ-layer forma- 
tion; intricate problems clearly elucidated in the frog’s case by aid of 
