296 PATTERNS AND PROBLEMS OF DEVELOPMENT 



play a part in determining the differences in different cells; but unless 

 the genie system is similar to Dreisch's entelechy, differences in gene effect 

 must be based on conditions in the cytoplasm. We can scarcely conceive 

 that substances produced by the genes can arrange themselves in an 

 orderly definite pattern resulting in the regional differentiation characteris- 

 tic of the individual and species, and there is nothing in nuclear pattern 

 that suggests the spatial pattern of differentiation. An orienting and or- 

 dering factor of some sort giving rise to a cytoplasmic pattern independ- 

 ently of the nucleus appears necessary for development. This pattern 

 must be the basis of physiological polarity, symmetry, asymmetry, and 

 of morphological form and differentiation in general. In preceding chap- 

 ters the presence of physiological gradients involving differences in meta- 

 bohc rate has been shown to be a feature of developmental pattern and 

 to be definitely related to the course of differentiation, but whether such 

 gradients are the primary factors of pattern or results of a still more funda- 

 mental pattern may still be questioned. 



An "intimate structure," an orientation of molecules or of colloidal par- 

 ticles crystalline in structure, or a space lattice have been assumed by 

 various authors to be the basis of developmental pattern. As regards such 

 hypothetical structures, it may be noted, first, that there is no evidence 

 of their existence as general characteristics of pattern. Orientation of mol- 

 ecules undoubtedly occurs in relation to interfaces, phase boundaries, etc., 

 and many highly differentiated structures give evidence of orientation of 

 molecules or particles; but there is no evidence that the polarity originat- 

 ing in a hydroid or planarian piece undergoing reconstitution results from 

 orientation of molecules in all the cells involved or from a space lattice 

 extending through the whole. It is difficult to believe that such a struc- 

 ture could persist even in eggs and embryos through all the changes in 

 position and form of cortical as well as other parts of the cytoplasm. 

 Second, if such structure exists, its orientation presumably originated in 

 relation to something. The free pole of the ovarian oocyte becomes the 

 apical or animal pole of the egg and embryo in many forms. If this polar- 

 ity consists in a molecular structure, how was that structure oriented in 

 that particular direction? That a metabolic differential is present between 

 free and attached poles of the oocyte is sufficiently evident in many cases 

 and appears beyond question in others. If such a differential is present, 

 it is highly improbable that a polar molecular orientation could occur in- 

 dependently of it; but, if it is the orienting factor, it, rather than the mo- 

 lecular orientation, is the primary polarity and the postulated molecular 



