THE PRESENT POSITION OF CELL-THEORY. 317 



parts of the nucleus, and on division, some of the daughter 

 nuclei may possess one set, others may possess another set 

 of " qualities ". By " qualities " I conceive that we mean 

 different chemical constitutions, and it would follow that the 

 daughter nuclei, being of diverse chemical constitutions, 

 would react in diverse manners on the adjacent protoplasm 

 and would each cause the delimitation of a territory of 

 cytoplasm within the limits of its own sphere of influence ; 

 in other words, cell bodies would be formed round nuclei of 

 different chemical constitutions. 



There is, however, yet another consideration to be taken 

 into account. As Hertwig has shown, the cytoplasm in 

 many ova is not homogeneous but is obviously separable 

 into tracts of unquestionably different chemical constitution. 

 This is conspicuously evident in the ova of Amphibia. As 

 the nucleus divides, its products come into relation with 

 different kinds of cytoplasm and the exchanges between 

 nucleus and cytoplasm will be different in different places 

 within the limits of the egg. Arguing on the same prin- 

 ciples as before, we may attribute the successive formation 

 of discrete blastomeres to this factor as much as to the 

 separation in the course of division of different qualities 

 contained in the egg nucleus, and according as one leans 

 towards an epigenetic or an evolutionary theory of develop- 

 ment so will one be disposed to lay more stress on the 

 one factor or the other. There is this much to be said, 

 that the most remarkable cell-lineages (which are only 

 traceable when the blastomeres are discrete) have been 

 observed in ova which contain a considerable proportion of 

 yolk, which is not evenly distributed throughout the egg, 

 and it is suggestive that segmentation in all cases leads to 

 the segregation of corpuscles richer in yolk from corpuscles 

 poorer in yolk — in fact to the segregation of materials of 

 diverse chemical constitution. 



Tempting as it is to pursue this subject further, I must 

 not attempt to do it now. But as I have claimed that the 

 views which I have tentatively put forward are agreeable 

 to the facts which we are in possession of, I may well give 

 a short summary of the facts which I have relied upon. 



