THE THEORY OF GERMINAL LOCALIZATION 397 



damental aspects as great a riddle as they were to the Greeks. What 

 we have gained is a tolerably precise acquaintance with the external 

 aspects of development. The gross errors of the early preformation- 

 ists have been dispelled. ^ We know that the germ-cell contains no 

 predelineated embryo ; that development is manifested, on the one 

 hand, by the cleavage of the ^gg, on the other hand, by a process of 

 differentiation, through which the products of cleavage gradually 

 assume diverse forms and functions, and so accomplish a physiological 

 division of labour. We can clearly recognize the fact that these pro- 

 cesses fall in the same category as those that take place in the tissue- 

 cells ; for the cleavage of the ovum is a form of mitotic cell-division, 

 while, as many eminent naturalists have perceived, differentiation is 

 nearly related to growth and has its root in the phenomena of nutri- 

 tion and metabolism. The real problem of development is the orderly 

 sequence and congelation of these pJic7iomena toward a typical result. 

 We cannot escape the conclusion that this is the outcome of the 

 organization of the germ-cells ; but the nature of that which, for lack 

 of a better term, we call ''organization," is and doubtless long will 

 remain almost wholly in the dark. 



In the following discussion, which is necessarily compressed within 

 narrow limits, we shall disregard the earlier baseless speculations, 

 such as those of the seventeenth and eighteenth centuries, which 

 attempted a merely formal solution of the problem, confining our- 

 selves to more recent discussions that have grown directlv out of 

 modern research. An introduction to the general subject may be 

 given by a preliminary examination of two central hypotheses about 

 which most recent discussions have revolved. These are, first, the 

 theory of Germinal Localization'^ of Wilhelm His ('74), and, second, 

 the Idioplasm Hypothesis of Nageli ('84). The relation between these 

 two conceptions, close as it is, is not at first sight very apparent ; 

 and for the purpose of a preliminary sketch they may best be con- 

 sidered separately. 



A. The Theory of Germinal Localization 



Although the naive early theory of preformation and evolution was 

 long since abandoned, yet we find an after-image of it in the theory 

 of germinal localization which in one form or another has been advo- 

 cated by some of the foremost students of development. It is main- 

 tained that, although the embryo is not \)X^formed in the germ, it must 

 nevertheless be Y>^Qdetermined in the sense that the <tgg contains 



^ Cf. Introduction, p. 8. 



2 I venture to suggest this term as an English equivalent for the awkward expression 

 " Organbildende Keimbezirke " of His. 



