972 THE AMERICAN NATURALIST. | [VOL. XXXIII. 
remains in the reader’s mind as to whether a fuller knowledge of the 
“ potency ” might make the supposition of the Aernkra/¢t unnecessary. 
Cannot formative material operated by chemical and physical forces 
yet furnish a solution of the problem? Before one is quite converted 
from the dogmatic materialism of the day to the rational vitalism of 
Driesch, it is fitting to know well the grounds of the new belief. 
The phenomena of localization are most admirably analyzed and 
discussed in the paper under consideration. Other general questions 
therein implied, as, for instance, Prospective Potency, are ere long to 
receive, we are informed, a fuller definition and discussion. These 
further studies will be awaited with much interest. 
Jonn H. GEROULD. 
Praxis und Theorie der Zellen- und Befruchtungslehre.!— 
This is a book of 260 pages, with 137 text-figures, and is designed 
to be a practical guide for courses in cytology. As such it cannot 
fail to be of service, since it is the first work yet published with this 
distinctive aim. There is, to be sure, in Bergh’s Zele und Gewebe 
an appendix on technique, but this is no more extensive than may 
be found in many text-books of histology, while the larger works of 
Hertwig, Wilson, and Henneguy present the facts and theories with 
regard to the cell from a general rather than from a laboratory 
standpoint. 
As the title indicates, the book also presents in brief form the 
general facts and theories of cytology, together with a short histori- 
cal review of different phases of the subject and references to some 
of the more important literature. This feature of the work, however, 
lacks the completeness and critical character of the larger works 
devoted to this field, while the laboratory directions are so inter- 
woven with the general part as to make the book much less readable 
than the other works mentioned. Unfortunately this combination 
also renders the book less useful as a laboratory guide, since the 
directions for preparing and observing material are less concise and 
explicit, and are much more difficult to find than would otherwise be 
the case. : 
The book is the outgrowth of a practical course on the cell given 
at the Zoölogical Institute at Freiburg, and in conformity with this 
course the contents are divided into sixteen days, two or three 
objects being considered each day. The subject for each day and 
the objects of study are as follows : First Day, Plant and animal 
3 Hacker, Dr. Valentin. Jena, Gustav Fischer, 1899, 7 marks. 
