NEW BIOLOGICAL BOOKS 



411 



of cells during gastrulation in the Amphib- 

 ian, and by Detwiler and others in 

 studying the fate of transplanted tissues. 

 The list is not complete, nor does it 

 necessarily cite in order of merit. 



Besides several digests in foreign lan- 

 guages, a number of summaries in English 

 are available. There may be mentioned 

 Jenkinson's Experimental Embryology 

 (1909), de Beer's succint and useful 

 Introduction to Experimental Embryology 

 (19x6) and, particularly, the masterly 

 chapters devoted to this subject in Pro- 

 fessor Wilson's The Cell in Development and 

 Inheritance (19x5). 



Since the publication of his work on 

 the development of the frog, Professor 

 Morgan has made a long and profitable 

 excursion into the field of genetics and has 

 become so intimately identified with this 

 field that many younger investigators 

 are apt to forget that he has been a prolific 

 contributor to experimental embryology 

 sensu strictu. In fact, he probably consid- 

 ers, and justly too, that he has been work- 

 ing in experimental embryology all the 

 time. 



And now Professor Morgan, after some 

 years spent in pursuit of the gene, presents 

 us with another stimulating review of 

 experimental embryology, which has 

 added importance in that it combines the 

 viewpoints of the embryologist and ge- 

 neticist. This time it is a volume of 766 

 pages containing a bibliography of 100 

 pages. It covers only the earlier phases 

 of development. He promises a future 

 volume which will consider the "more 

 obviously physiological changes" of de- 

 velopment, growth, reflex actions, tro- 

 pisms, sex determination, embryonic 

 grafting, influence of environment on the 

 embryo, the source of the energy of 

 development, etc. 



As a whole, the present volume may be 

 characterized as an eminently readable sum- 



mary and analysis of the outstanding 

 developments in the field surveyed. There 

 are twenty-five chapters. The first is an 

 interesting discussion of the experimental 

 method. This is followed by seven chap- 

 ters dealing with the problems connected 

 with fertilization. There are others de- 

 voted to the physico-chemical changes in 

 the egg after fertilization, the mechanism 

 of cleavage, the establishment of sym- 

 metry and the origin of asymmetry, 

 localization before cleavage and the re- 

 distribution of the visible materials of the 

 egg by centrifuging, the development of 

 whole or partial embryos from isolated 

 blastomeres, the mechanics of organ 

 formation, the fate of cells and their 

 location, the fusion of two eggs to produce 

 one embryo. There is a long chapter on 

 artificial parthenogenesis and three chap- 

 ters are devoted to the chromosomes of 

 the egg and their division, the Mendelian 

 inheritance of embryonic and larval char- 

 acters and the development of species 

 hybrids. 



It is only natural that the chromosome 

 and the gene figure prominently in the 

 book. Professor Morgan has always 

 realized that an adequate theory of hered- 

 ity must at the same time offer a satisfac- 

 tory explanation of the differentiation of 

 the embryo. In discussing this question 

 in The Mechanism of Mendelian Heredity 

 in 192.3 he stated (p. x8o) that 



The cause of the differentiation of the cells of the 

 embryo is not explained on the factorial hypothesis 

 of heredity. On the factorial hypothesis the factors 

 are conceived as chemical materials in the egg, which, 

 like all chemical bodies, have definite composition. 

 The characters of the organism are far removed, in all 

 likelihood, from these materials. Between the two 

 lies the whole world of embryonic development in 

 which many and varied reactions take place before the 

 end result, the character, emerges. Obviously, 

 however, if every cell in the body of one individual 

 has one complex and every cell in the body of another 

 individual has another complex that differs from the 

 former by one difference, we can treat the two sys- 



