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THE AMERICAN NATURALIST [Vol. XLII 



assigned to them can not be denied, and this is the simplest in- 

 terpretation that embryologists have given to their results; but 

 it is improbable that the differences between whole and partial 

 development rest on this condition alone. The phenomena are 

 really more complicated, as shown when whole development oc- 

 curs even after the materials of the egg have become shut up 

 within cells, and as shown in regeneration when a whole animal 

 of smaller size develops from a part of the body of a fully formed 



The final chapter deals with the influence of external factors, 

 and, although compressed into very small compass, most of the 

 modern work is referred to at least briefly. The author's con- 

 clusion that external factors play in most cases only a minor 

 role in development will be conceded by most students of ex- 

 perimental embryology. Nevertheless, important results have 

 been already obtained from a study of external factors and our 

 causal knowledge of the physiology of development rests in the 

 main on evidence from such studies. Whether the formative 

 changes are simply a complex of physiological relations or de- 

 pend on factors not usually considered by physiologists is at 

 present a question of opinion rather than of demonstration. 



While we have found ourselves obliged in many cases to urge 

 that many of the conclusions reached by the author still remain 

 uncertain, and in other cases to take issue with the author's sum- 

 maries of the present state of affairs, yet it should, be repeated 

 that the chief value of the book lies less in such generalizations 

 than in the conscientious, full and exact review of the literature. 

 The results so far obtained are marshaled forth in excellent 



will be found useful not only to those who do not have access 

 to the original literature or time to digest it, but also to those 

 who have felt themselves somewhat overwhelmed by the rapid 

 development of this branch of embryology. The general reader, 

 too, will find in this volume a well-balanced summing up of the 

 more important results of the school of experimental embryology. 



M. 



Jordan on Fishes. 1 — In a single large volume President Jordan 

 presents ''virtually all the non-technical material contained in 



