4 zz 



THE QUARTERLY REVIEW OF BIOLOGY 



tems as two complexes quite irrespective of what 

 development does so long as development is orderly. 

 It is sometimes said that our theories of heredity 

 must remain superficial until we know something of 

 the reactions that transform the egg into the adult. 

 There can be no question of the paramount importance 

 of finding out what takes place during development. 

 The efforts of all students of experimental embryology 

 have been directed for several years toward this goal. 

 It may even be true that this information, when 

 gained, may help us to a better understanding of the 

 factorial theory, we cannot tell; .... Although 

 Mendel's law does not explain the phenomena of 

 development, and does not pretend to explain them, 

 it stands as a scientific explanation of heredity, be- 

 cause it fulfills all the requirements of any causal 

 explanation. 



He makes substantially the same state- 

 ment in his Theory of the Gene (19x6, p. 2.6). 

 But in spite of the fact that he disclaims 

 any pretension to explain differentiation 

 on the basis of the gene theory, Professor 

 Morgan usually gives some indication of 

 his feelings on the subject. For instance, 

 in 19x3, he made this statement (p. 43): 

 "Mendelian factors are not sorted out 

 .... differentiation is due to the cu- 

 mulative effect of regional differences in the 

 egg and embryo, reacting with a complex 

 factorial background that is the same in 

 every cell." 



It was with particular interest, therefore, 

 that we examined this latest book of his 

 to ascertain whether his ideas on this 

 matter had been clarified and whether in 

 this book dealing primarily with experi- 

 mental embryology he would attempt to 

 bring the theory of the gene into some 

 closer harmony with the observed course 

 of embryonic differentiation. It will per- 

 haps be helpful to summarize in the 

 briefest fashion what Professor Morgan 

 has termed the theory of the gene, with 

 special reference to the problem of differ- 

 entiation and disregarding such aspects as 

 linkage, etc. The theory states that the 

 characters of the individual are repre- 

 sented in the chromosomes of the egg by 



minute entities termed the genes, which 

 are arranged in linear order. Each gene 

 may affect the development of many char- 

 acters and the interaction of all the genes 

 may be involved in the manifestation of 

 any particular character. The genes are 

 present in pairs, the members of each pair 

 separating during maturation of the germ 

 cell, but during the ordinary mitoses each 

 gene is divided equally so that the daugh- 

 ter cells are factorially identical. It may 

 be added that cytological evidence seems 

 to indicate positively that the somatic 

 mitoses are quantitative so far as the 

 nucleus is concerned and most observers 

 accept this as an establish fact. The 

 problem, then, is how, with identical 

 factorial equipment, differentiation is 

 started, and how, when once initiated, it 

 is brought to completion. 



There would seem to be at least three 

 factors to be considered, (1) the nucleus, 

 (z) the cytoplasm, (3) the environment. 

 Investigators have attempted to answer 

 the question in different ways. The 

 importance of the cytoplasmic localiza- 

 tions of the egg have been stressed, the 

 external environment of the developing 

 egg has been emphasized as important, and 

 for others the nucleus is the most impor- 

 tant element. For instance, Child has 

 expressed himself as follows: . . . . 

 development of the individual represents 

 the reaction of the factorial complex to 

 environmental factors. This seems to be 

 Conklin's viewpoint C 2 - 2 -) an( i it is essen- 

 tially the viewpoint of the present book, 

 according to which the physiological 

 gradient constitutes the primary regional 

 differential, to which the factorial com- 

 plex reacts." Conklin (In Cowdry, Gen- 

 eral Cytology, 'z4, p. 600) emphasizes the 

 importance of the cytoplasm as follows : 



Given a definite polarity, symmetry, and pattern 

 of the egg, all other differentiations of ontogeny 

 could be explained as due to the interaction of non- 



