Embryology. 139 



original cell had the qualities abcxyz, it is supposed that 

 its two daughter-cells might have the qualities abcxy 

 and abcxz. And what each cell becomes, is from the 

 first determined by the particular contingent of vital 

 qualities with which it starts. 



According to the anti-mosaic theory, cell-division is 

 quantitative , i.e. without any sifting out of vital units, 

 and the cause of differentiation is to be found in the 

 varied relations in which the cells find themselves. The 

 prospective value of embryonic cells, Driesch says, is 

 " a function of their location ". Each of the early cells 

 is supposed to have a complete set of specific charac- 

 teristics, but some remain latent while others become 

 active, this being determined by the relations of the 

 particular cell to the whole of which it forms a part. 



These two theories, over which a long-drawn-out 

 battle has been fought, agree in recognizing a complex 

 organization in the ovum. Although we cannot see it, 

 or even imagine it, there must be in the egg a complex 

 architectural arrangement of some sort, corresponding 

 to the hereditary qualities. The two theories differ as 

 to the manner in which differentiation occurs, the first 

 relying on the hypothesis of qualitative division, the 

 second on the hypothesis of cellular interaction. 



The two most serious objections to the mosaic theory 

 are: (i) that there is no proof forthcoming of qualitative 

 cell-division ; and (2) that an isolated cell from the 2-cell 

 or 4-cell stage of a developing ovum may, in many cases 

 (lancelet, sea-urchin, &c.), give rise to an entire embryo. 



The most serious objections to the anti-mosaic theory 

 are found in those cases where even the first cleavage of 

 the egg results in two unequal cells, as in Nereis, the 

 reason for this being some unknown predetermination 

 within the ovum. 



