ON THE NA TURE AND CA USES OF DIFFERENTIA TION 3 1 1 



below a larger clear macromere in which the oil-drops remain. 

 The sixteen-cell stage, therefore, consists of eight deutoplasm-laden 

 macromeres and eight protoplasmic micromeres (instead of four 

 macromer.es and twelve micromeres, as in the usual development). 

 These embryos developed into free-swimming trochophores contain- 

 ing eight instead of four macromeres, which have the typical clear 

 protoplasm containing oil-drops. In this case there can be no doubt 

 whatever that four of the entoblastic nuclei were normally destined 

 for the first quartet of micromeres (Fig. 136, B), from which arise the 

 apical ganglia and the prototroch. Under the conditions of the 

 experiment, however, they have given rise to the nuclei of cells 

 which differ in no wise from the other entoderm-cells. Even in a 

 highly differentiated type of cleavage, therefore, the nuclei of the 

 segmenting egg are not specifically different, as the Roux-Weismann 

 hypothesis demands, but contain the same materials even in cells that 

 undergo the most diverse subsequent fate. But there is, furthermore, 

 very strong reason for believing that this may be true in later stages 

 as well, as Kolliker insisted in opposition to Weismann as early as 

 1886, and as has been urged by many subsequent writers. The strong- 

 est evidence in this direction is afforded by the facts of regeneration ; 

 and many cases are known for instance among the hydroids and the 

 plants in which even a small fragment of the body is able to repro- 

 duce the whole. It is true that the power of regeneration is always 

 limited to a greater or less extent according to the species. But there 

 is no evidence whatever that such limitation arises through specifica- 

 tion of the nuclei by qualitative division, and, as will appear beyond, 

 its explanation is probably to be sought in a very different direction. 



F. ON THE NATURE AND CAUSES OF DIFFERENTIATION 



We have now cleared the ground for a restatement of the prob- 

 lem of development, and an examination of the views opposed to the 

 Roux-Weismann theory. After discarding the hypothesis of quali- 

 tative division the problem confronts us in the following form. If 

 chromatin be the idioplasm in which inheres the sum-total of heredi- 

 tary forces, and if it be equally distributed at every cell-division, how 

 can its mode of action so vary in different cells as to cause diversity 

 of structure, i.e. differentiation ? It is perfectly certain that differen- 

 tiation is an actual progressive transformation of the egg-substance 

 involving both physical and chemical changes, occurring in a definite 

 order, and showing a definite distribution in the regions of the egg. 

 These changes are sooner or later accompanied by the cleavage 

 of the egg into cells whose boundaries may sharply mark the 



