WEISMANNISM AND OTHER THEORIES 411 



Non-factorial Theories. The above theory of the dependence of t he 

 course of development upon the operation of an internal factorial mech- 

 anism is essentially an "elementalistic" conception: the attempt is 

 made to explain the organism in terms of its constituent parts, namely, 

 the cells and smaller elements contained by them. As noted in our 

 historical sketch, a number of botanists and zoologists many years ago 

 called attention to the fact that limits must be set to the conception of 

 the cell as the unit of structure and function ; and they have been followed 

 by a school, made up largely of experimental embryologists, which holds 

 that organization is not the result of cell formation, but rather precedes 

 and regulates the latter. From this " organismal " standpoint the or- 

 ganism as a whole, and not one or another of its elementary parts, is 

 regarded as the primary individual. This individual is something more 

 than the cell aggregate pictured by Schleiden and Schwann: it dominates 

 the activity of its constituent members from the beginning of the life 

 cycle onward, and behaves as a unit irrespective of the manner and degree 

 of its subdivision into special centers of action, the cells. The condition 

 present in ccenocytic plants is especially noteworthy in this connection, 

 as are also those cases among animals in which a derangement of the 

 early embryonic cells does not prevent the eventual attainment of the 

 normal form. As urged with much force by Ritter (1919), "the organism 

 in its totality is as essential to an explanation of its elements as its 

 elements are to an explanation of the organism." 



The factorial theory may also be said to represent preformationism 

 in a very modern form. "We are sailing nearer the preformation coast 

 than at any time since the modern study of development began under 

 von Baer" (Conklin 1913). Directly opposed to corpuscular and fac- 

 torial theories of development are those which seek to explain the course 

 of ontogenesis not by an internal mechanism but rather as the result of 

 the influence of external agencies and the physiological responses shown 

 by protoplasm in the form of cells to such influence: development is 

 held to be truly epigenetic. The control exercised by environmental 

 factors during the organism's early developmental stages, and the effects 

 of various tropisms and tactisms between the component cells upon the 

 type of organization resulting, have been especially emphasized by O. 

 Hertwig, Hartog, Roux, Herbst, Driesch, and others. The most sug- 

 gestive recent work of this nature in plants is that of Harper (1908, 

 1918a6) on colonial algae. In Hydrodictyon and Pediastrum a number of 

 free-swimming cells come together and build up colonies of very definite 

 forms, and a series of experiments has shown that the position in the colony 

 of any given cell is in no way predetermined. As already pointed out 

 in Chapter XIV, Harper contends that the type of multicellular organiza- 

 tion thus built up in successive life cycles is to be explained as the result 

 of physico-chemical interactions between independent cells organized as 



