284 



SCIENCE. 



[N. S. Vol. VII. No. 165. 



to a mucli later chapter. The discussion at 

 this point takes the form of an illustrated ac- 

 oount of Koux's experiments on the form and 

 arrangement of oil-drops divided into parts 

 similar to the blastomeres of the egg. The 

 conclusion is drawn that surface tension is an 

 important factor in the arrangement of cleaving 

 cells, but that these are influenced also by 

 many other factors which prevent them from 

 showing always the typical arrangement de- 

 manded by surface tension alone. 



Chapters V. and VI. are devoted to a descrip- 

 tive account of the developmental processes 

 from cleavage until after the establishment of 

 the germ layers. The formation of the embryo 

 by concrescence makes the basis of an exception- 

 ally clear and satisfactory description of the 

 complicated processes taking place. In this 

 connection is given a brief statement of the ex- 

 perimental evidence (formation of extra-ovates, 

 etc.) of the changes taking place, and of the 

 correspondence of particular parts of the egg 

 with parts of the later embryo, and the attempts 

 of His to explain many of the processes of 

 development by means of experiments with 

 elastic plates are outlined. 



The nucleus of the book is formed by Chap- 

 ters VII. to XII., which are devoted to the ex- 

 periments for which the frog's egg has served 

 as the most frequent object in the study of 

 early developmental processes. This account 

 of experimental work is not brought into any 

 close connection with the foregoing description 

 of the normal development of the frog. The 

 latter is closed oiF, up till after the formation of 

 the germ layers, then the experimental work on 

 early stages is taken up. The account of this is 

 classified only loosely according to the pro- 

 cesses and problems studied, the arrangement 

 adopted being chiefly a historical one. 



Chapter VII. gives an account of the experi- 

 mental production of embryos with spina bifida, 

 with especial reference to its bearing on the 

 formation of the embryo by concrescence of the 

 two halves of the germ ring. 



Chapters VIII. to XI. are devoted to an 

 account, arranged chiefly historically, of the ex- 

 periments of Pfliiger, Boru, Roux and many 

 later investigators on the modifications in de- 

 velopment induced by an altered relation of the 



egg to the direction of gravity, by compression 

 and by killing or isolating individual blas- 

 tomeres in early stages. This forms one of the 

 most instructive chapters in the history of bio- 

 logical investigation and theory, illustrating 

 and emphasizing, as it does, the necessity for 

 extreme caution in generalizing the results of 

 experiments and observations on single forms, 

 and showing how false may be the conclusions 

 based upon the clearest evidence when that 

 evidence is not gathered from extensive com- 

 parative researches. The lesson thus gained 

 has been of the greatest importance and has 

 doubtless been one of the most valuable results 

 of this series of investigations. It would be 

 interesting to review here, following Professor 

 Morgan, the problems proposed, the experiments 

 undertaken to answer these questions, the con- 

 clusions drawn from these experiments, and 

 the continued modification of these conclusions 

 as the circle of experimentation became wider. 



The history of the development of opinion as 

 to the conclusions to be drawn from the ' total ' 

 or ' partial ' development of isolated blasto- 

 meres, of the theories concerning the part played 

 by gravity in cleavage, and of the general fac- 

 tors determining the direction and position of 

 cleavage planes, is remarkably instructive. Al- 

 most more important, as leading to more defi- 

 nite positive conclusions, is the history of the 

 gradual change from the view that the nucleus 

 is the all-important factor in formative processes, 

 to that which seeks the essential factors in the 

 cytoplasm, culminating with Driesch and Mor- 

 gan's demonstration that in the ctenophore 

 purely cytoplasmic injuries to the egg result in 

 corresponding modifications in the larva. But 

 for a full discussion of these and other matters 

 the reader must be recommended to a perusal 

 of the book itself. 



A few words may be added as to Professor 

 Morgan's own conclusions in regard to some of 

 the problems discussed. Although he states in 

 the preface that he has avoided the discussion 

 of theoretical questions, as out of place in such 

 a book, he does give his views on a number of 

 important points. 



In Chapter XII., 'Interpretations and Conclu- 

 sions,' a clear and appreciative survey is given 

 of Roux's profound analysis of the problems of 



