Nature of Heredity. 335 



A striking illustration of the manner in which the equilibration 

 between the constituent cells of the body is maintained is afforded 

 by the experiments of Hans Driesch and others upon developing 

 eggs. To quote from Professor Minot : — " The egg of a sea-urchin 

 divides into two cells, each of which multiplies and normally gives- 

 rise to half of the body of the animal. By somewhat violent shaking, 

 the two cells may be artificially separated ; each cell may then develop 

 into a complete larval sea-urchin, but of half the normal size only. 

 Similar experiments have since been made by other investigators^ 

 who have obtained like results with other animals, vertebrate as well 

 as invertebrate. Even more remarkable larvae have been raised 

 from blastomeres of the four-cell and eight-cell stages of seg- 

 mentation, producing larvae of one-fourth and one-eighth the normal 

 size." 



Now. if each blastomcre when isolated from its fellows developes- 

 into a complete larval animal, why does it not do so when it remains 

 in contact with its fellows? It can be only the mutual influence of 

 the blastomeres upon one another which determines what part each 

 shall play in the development at this early stage. By virtue of the 

 karyokinetic process we may suppose that the first division of the 

 nucleus of the egg results in the formation of two centres of control 

 of equal dynamic value, each, therefore, requiring the same form of 

 cell-body to equilibrate it. If the blastomeres are separated from 

 one another each will divide again in the same way because it has 

 the same dynamic value. If, however, they remain in contact, as 

 they normally do, the dynamic value of each is modified by the 

 presence of the other, and they will behave accordingly. The 

 ultimate form of the multicellular body to which they give rise will be 

 the resultant of the forces exerted by all the blastomeres and their 

 nuclei as they continue to divide, modified to some extent, no doubt, 

 by the physical environment. 



Thus it is that each artificially separated blastomere gives rise 

 to a complete but diminutive larva, the same result being attained by 

 the action of identical forces in each case, and this being the only 

 result which can equilibrate the forces at work. 



It may be urged that the difference in behaviour between an 

 isolated blastomere and one which remains in contact with its fellows 

 is due to mechanical causes, such as pressure, etc., in other words, 

 that it may be external and not internal forces which determine the 

 behaviour of the blastomeres. To some extent no doubt this is true, 

 but this in no way affects the proposition that the forces, presumably 

 centred in the nuclei of the blastomeres, require at every successive 

 stage of development a certain definite form and arrangement of the 

 blastomeres to equilibrate them. Moreover, a different tvpe of 

 arrangement is characteristic of different types of organisms, which 

 proves conclusively that internal as well as external forces are con- 

 cerned in producing the result. At any rate we see clearly that when 

 the equilibrium existing between the constituent cells or blastomeres 

 of an embno is upset by separating them, it mav be restored again 



