424 HEREDITY AND DEVELOPMENT 



adapted to secure the exact equivalence of the two daughter- 

 nuclei. It may be, however, that while there is always a general 

 equivalence, in the sense, for instance, that the large nuclear 

 bodies or chromosomes are accurately split, and that each 

 daughter-cell gets the same number, there may be at the same 

 time a more intricate qualitativeness in the division. Again, 

 the critics have brought forward some of the results of experi- 

 mental embryology which seem at first sight to tell against the 

 hypothesis of differential division, especially where one of the 

 first two or first four blastomeres is seen to form a complete 

 and normal embryo, or where under artificial conditions (of 

 pressure, etc.) certain cells develop into tissues which in normal 

 conditions are formed by quite different cells. To explain these 

 and other difficulties — e.g. regenerative phenomena — various 

 ingenious sub-hypotheses have been invented. It seems highly 

 probable that the distribution of particular characters (if it be 

 a reality at all) occurs sooner in some developing eggs than in 

 others ; in other words, that the cells of some embryos are " set " 

 and defined at an earlier date than those of other embryos. 



Non-Mosaic Theories. — All embryologists agree that a germ- 

 cell has a specific organisation, but many will not admit that 

 it is necessary or useful to people the nucleus with a large body 

 of representative particles, ready to distribute themselves and 

 work upon the virgin soil which the protoplasm affords. All 

 agree that there is gradual differentiation of cells as development 

 proceeds, but many will not admit that it is necessary or useful to 

 think of this in terms of a distribution of representative particles 

 from the original depot in the nucleus of the fertilised egg-cell. 



Oscar Hertwig may be named as a prominent representative 

 of those who give the facts of development an interpretation 

 somewhat different from that suggested by Roux and Weismann. 

 We may suppose that, from the youngest ovarian ovum onwards, 

 the nucleus exerts a " control " upon the surrounding cytoplasm, 

 whether by the migration of " pangens " (De Vries), or of 



