264 GENERAL EMBRYOLOGY 



shown clearly that cell interactions alone play but a small 

 part indeed in the process of differentiation, and has led to the 

 search for that underlying factor or group of factors. And 

 while Driesch himself concludes that no explanation is possible 

 in known terms of matter or energy, and relies upon an unknown, 

 and therefore metaphysical, factor, the great majority of em- 

 bryologists believe that the question is still susceptible of 

 further scientific analysis. We find two general hypotheses 

 regarding the nature of the causes or conditions of differentia- 

 tion, and since differentiations are always specific we may speak 

 of these as also hypotheses as to the causes of those resemblances 

 among generations of organisms which we call in a word, 

 heredity. 



The first of these is the hypothesis of "germinal localization" 

 or "germinal, organ-forming regions" associated primarily 

 with the names of His, Lankester, and Whitman. The essen- 

 tials of this hypothesis in its present form may be stated as 

 follows. The cytoplasm of the ovum before development (i.e., 

 cleavage) begins, has a definite structure or morphology of its 

 own, such that particular regions or substances, by effecting 

 specific developmental reactions, are seen to correspond with, 

 or to lead to the formation of, particular tissues -or structures 

 of later stages and of the fully developed organism. The 

 cytoplasm is conceived as a mosaic-work of physiological units 

 which have not only a definite morphology but a definite pro- 

 morphology, looking toward the structure of the mature individ- 

 ual. This immediately suggests the old idea of "preforma- 

 tion" but it omits, of course, the naive crudities of this con- 

 ception and rests upon the idea that certain structures and 

 regions of the egg cytoplasm are in direct genetic relation with 

 corresponding structures and regions differentiating later by a 

 true process of development or epigenesis. These germinal 

 structures have specific reference, but not resemblance, to the 

 parts of the mature organism. This predetermination may be 

 only general to begin with, but it becomes more complete and 

 specific as one condition succeeds another, the cytoplasmic 

 structure of the ovum representing in the beginning all there 



