8 BIOLOGICAL LECTURES. 



of differentiation, for there is no sifting apart of the idioplasmic 

 units, but an equal distribution of them to all the cells of the 

 body. In other words, the cleavage of the ovum does not effect 

 an analysis of the idioplasm into its constituent elements, but 

 only breaks it up into a large number of similar masses. Dif- 

 ferentiation follows upon cell-division, is caused by the inter- 

 action of the parts of the embryo, and the character of the 

 individual cell is determined by its environment — /. e., by its 

 relation to the whole of which it forms a part. "The ^gg,'' 

 says Hertwig, " is an organism, which multiplies by division 

 to form numerous organisms equivalent to itself, and it is 

 through the interactions of all these elementary organisms, at 

 every stage of the development, that the embryo, as a whole, 

 undergoes progressive differentiation. The development of a 

 living creature is therefore in no wise a mosaic work, but, on 

 the contrary, all the individual parts develop in constant rela- 

 tion one to another, and the development of the part is always 

 dependent on the development of the whole." There is there- 

 fore no necessary relation between the individual blastomeres 

 of the segmenting ovum and the parts of the adult body to 

 which they give rise ; this relation is purely fortuitous. The 

 most extreme statement of this view appears in the writings of 

 Pfliiger and Driesch. " I would accordingly conceive," says 

 Pflu"er, '• that the fertilized <i^ig has no more essential relation 

 to the later organization of the animal than the snowflake has 

 to the size and form of the avalanche which, under appropriate 

 conditions, may develop out of it." Driesch, writing ten years 

 later ('89), is no less explicit. He regards the blastomeres of 

 the lichiiuis embryo, as "composed of an indifferent material, 

 so that they may be thrown about at will, like balls in a pile, 

 without the least impairment of their power of development." 

 The ultimate fate of any particular blastomere is determined 

 by its relative position in the mass ; that is (to quote his own 

 striking aphorism), "their prospective value {Bcdcutung) is a 

 function of their location " (cf. His). 



We shall presently return to these more extreme views, but 

 1 will here point out one all-important point which is definitely 

 established by the work of Driesch and other experimentalists, 



