378 THE EVOLUTION THEORY 



In iiuiiiy aiiiiiials the fertili/i'd ovum dividcR at the first seg- 

 iiu'iitation into two colls, one of which gives rise predominantly 

 to the outei-, llie other to the inner germinal layer, as in molluscs, for 

 instance. Let us now assume that this is the case altogether, so that 

 one of the first two blastomeres gives rise to the whole of the ectoderm, 

 the other to the whole of the endoderm ; we should here have a differ- 

 ential division, for the developmental import (the ' prospective ' of 

 Driesch) of the primitive ectoderm-cell is quite different from that 

 of the primitive endoderm-cell, the former giving origin to the skin 

 and the nervous system, with the sense organs, while the second gives 

 rise to the alimentary canal, with the liver, &c. Through this step 

 in segmentation, I conclude, the determinants of all the ectoderm- 

 cells become separated from those of the endoderm-cells ; the 

 determinant architecture of the ids must be so constructed in such 

 species that it can be segregated at the first egg-cleavage into ecto- 

 dermal and endodermal groups of determinants. Such differential 

 divisions will always occur in embryogenesis when it is necessary 

 to divide a cell into two daughter-cells having dissimilar develop- 

 mental import, and consequently they will continue to occur until the 

 determinant architectvire of the ids is completely analysed or segre- 

 gated out into its different kinds of determinants, so that each cell 

 ultimately contains only one kind of determinant, the one by which 

 its own particular character is determined. This character of course 

 consists not merely in its morphological structure and chemical con- 

 tent, but also in its collective physiological capacity, including its 

 power of division and duration of life ^. 



But embryogenesis does not proceed by differential divisions 

 alone, for integral divisions are often interpolated between them, 

 always, for instance, when in a bilateral animal an embryonic cell 

 has to produce by division into two a corresponding organ for the right 

 and left sides of the body ; for instance, in the division of the primi- 

 tive genital cell into the rudiments of the right and left reproductive 

 organs, or in the division of the primitive mesoderm-cell into the right 

 and left initial mesoderm-cell, but also later on in the course of embryo- 

 genesis, when, for instance, the right or the left primitive reproductive 



' Emery has lately called attention to anothei- direct i^roof of the existence of 

 differential cell- and nucleus-division. According to observations made by Giardina, 

 in the water-beetle {Dytiscus), one primitive ovum-cell gives rise, through four successive 

 divisions, to fifteen nutritive cells and one well-defined ovum-cell. But only half of 

 the nuclear substance takes part in these divisions, the rest remains inactive in 

 a condensed, cloudj' condition. ' The meaning of the whole process is obviously that 

 the germ-plasm mass as a whole is handed over to the ovum-cell, while the nutritive 

 cells receive only the nuclear constituents which belong to them' {Biol. CenimlbL, May 

 15, 1903)- 



