358 DYNAMICS OF HISTOGENESIS. I 



Lewis (1910) endeavored to solve the problem at how early a period 

 in the development of the ovum this power of self-differentiation of 

 muscle tissue begins. He found by transplanting tissue from the lips 

 of the blastopore in the early gastrula stage of the frog that this tissue 

 later on showed muscular differentiation. The conclusion is drawn 

 "that muscle tissue is already predetermined in the early gastrula." 



The idea conveyed by the last statement is that muscular tissue is 

 formed, sui generis, by some inherent predetermination and not by 

 the agency of its surroundings nor due to its position in the whole. 

 Lewis' view-point is in accord with Conklin's (1905) as seen in the 

 following statement of the latter observer: "The potencies or pro- 

 spective values of any blastomere are not primarily a function of its 

 position, but rather of its material substances." 



There are three theories regarding cellular differentiation; first, the 

 "mosaic theory" of Roux (1881), later modified by Wilson (1904), 

 Conklin (1905), Zeleny (1904), and Boveri; second, the "organization 

 theory" of Whitman and more recently elaborated by Child (1915) 

 in his studies on metabolic gradients and individuality; third, "the 

 homogeneity theory" of Driesch (1894, 1899). Driesch considers the 

 peculiar organizing quality of protoplasm as due to the expression of 

 a mysterious force wholly different from any in the inorganic world. 



His, Roux (1881, 1892, 1893), Wilson (1892, 1893, 1897, 1904), and 

 Conklin (1905) lay emphasis upon the cell as the key to all ultimate 

 biological problems. Whitman, on the other hand, points out the 

 inadequacy of the cell theory to development. "That organization 

 precedes cell formation and regulates it, rather than the reverse, is 

 a conclusion that forces itself upon us from many sides," is a sum- 

 mary of his studies. Morgan (1895, 1898) had deduced the idea 

 from his studies on regeneration that the multicellular individual is a 

 whole in the same sense that the unicellular form is a whole. Child 

 (1899, 1915) also lays emphasis on the fact that it is the "organism — 

 the individual, which is the unit and not the cell." Differentiation of a 

 single cell, consequently, according to Child and Whitman, is a func- 

 tion of its position in the whole. This view is also upheld by Driesch 

 (1894). Wilson and Conklin, on the other hand, conclude that po- 

 tencies are functions of the material substance of the cell. 



