120 SCIENCE PROGRESS. 



fresh cleavage, owing to the increased reaction of the more 

 numerous constituents of the cell aggregate. 1 



I am not sure that I have quite fairly represented the 

 views of any single author. Herbert Spencer certainly 

 considered that the physiological units, whose existence 

 he postulated, were all alike in kind in each individual 

 organism, and that the germ cell, from which the organism 

 is derived, contained small groups of these units. The 

 difference in the arrangement of units alike in kind deter- 

 mines the diversity of the parts of the body, according to his 

 theory ; the diversity in the constitution of the units being 

 the cause of the distinction between different species and 

 different individuals. On this view units are physiologically 

 variable quantities, which act under the directing influence 

 of the whole organism. This is very similar to Driesch's 

 statement that the prospective character of each cell is a 

 function of its location. Recent writers are not so explicit 

 in the statement that all the units composing the individual 

 are alike in kind. The position is very much altered if it 

 is assumed, as they seem to assume, that the units com- 

 posing the individual are of many different kinds, of as 

 many kinds as there are different kinds of cells in the adult 

 (in so far agreeing with Weismann and Roux), but that in 

 each cell divisions all the kinds of units are transferred from 

 mother to daughter cell. In such a case every cell w r ould 

 have primitively the power of going through the whole 

 course of ontogeny, thus accounting for the phenomena of 

 regeneration and reformation, whilst the " function of the 

 location " would be limited to determining- which of the units 

 present should impress on the cell the character necessitated 

 by its position in the organism. This is the view taken by 



1 "As the ontogeny advances, the idioplasm of the cells undergoes 

 gradual and progressive physiological modifications (brought about by the 

 interaction of the various parts of the embryo), without, however, losing 

 any of its elements. The isolation of a blastomere restores it in a measure 

 to the condition of the original ovum, and the idioplasm, therefore, tends 

 to return to the condition of the original germ plasm, and thus to cause a 

 repetition of the development from the beginning " (E. B. Wilson, " Am- 

 phioxus and the Mosaic Theory ," Journal of Morphology, vol. viii.,p. 609). 



