300 THEORIES OF INHERITANCE AND DEVELOPMENT 



are expressed with great caution and with a full recognition of the 

 difficulty and complexity of the problem. From his latest essay, in- 

 deed ('94); it is not easy to gather his precise position regarding the 

 theory of cytoplasmic localization. Through all his writings, never- 

 theless, runs the leading idea that the germ is definitely organized 

 before development begins, and that cleavage only reveals an organ- 

 ization that exists from the beginning. "That organization precedes 

 cell-formation and regulates it, rather than the reverse, is a conclu- 

 sion that forces itself upon us from many sides." ^ "The organ- 

 ism exists before cleavage sets in, and persists throughout every 

 stage of cell-multiplication."^ In so far as this view involves the 

 assumption that the organization of the egg-cytoplasm at the be- 

 ginning of cleavage is a primordial character of the egg, Whitman's 

 conception must, I think, be placed on the side of the localization 

 theory ; but his point of view can only be appreciated through a 

 study of his own writings. 



All of these views, excepting those of Roux, lean more or less 

 distinctly towards the conclusion that the cytoplasm of the egg-cell 

 is from the first mapped out, as it were, into regions which corre- 

 spond with the parts of the future embryonic body. The cleavage 

 of the ovum does not create these regions, but only reveals them to 

 view by marking off their boundaries. Their topographical arrange- 

 ment in the o.^'g does not necessarily coincide with that of the adult 

 parts, but only involves the latter as a necessary consequence — some- 

 what as a picture in the kaleidoscope gives rise to a succeeding pic- 

 ture composed of the same parts in a different arrangement. The 

 germinal localization may, however, in a greater or less degree, fore- 

 shadow the arrangement of adult parts — for instance, in the egg of 

 the tunicate or cephalopod, where the bilateral symmetry and antero- 

 posterior differentiation of the adult is foreshadowed not only in the 

 cleavage stages, but even in the unsegmented t2,g. 



By another set of writers, such as Roux, De Vries, Hertwig, and 

 Weismann, germinal localization is primarily sought not in the cyto- 

 plasm, but in the nucleus ; but these views can best be considered 

 after a review of the idioplasm hypothesis, to which we now proceed. 



B. The Idioplasm Theory 



We owe to Nageli the first systematic attempt to discuss heredity 

 regarded as inherent in a definite physical basis ; ^ but it is hardly 

 necessary to point out his great debt to earlier writers, foremost 

 among them Darwin, Herbert Spencer, and Hackel. It was the 



1 '93, p. 115. - /.<■., p. 112. 3 j'/igorie der Abstaviinungslehre, 1884. 



