90 Geometrical relation of Nuclei 



different types of bilaterally symmetrical forms will be attained. 

 It is characteristic of the Chordata (excepting probably the 

 Enteropneusts) that the ventral part of the deuterogenetic centre 

 dies out early, so that growth in length in that phylum is chiefly 

 attributable to the dorsal part of the original blastopore lip. This 

 dorsal part is the part of the lip, which is earliest formed in 

 Amphibians, and as it begins its growth at once, before the whole 

 archenteron is completed, the correct understanding of the pro- 

 cess has been a matter of some difficulty, and has been arrived at 

 only after much research and not a little controversy. 



No doubt the denial of ids and engrams during development 

 only puts back the difficulty to the previous generation, to the 

 building up of the germ cells. But there is this difference. The 

 potter turns out jar after jar exactly alike not because the clay 

 contains an id or engram that causes the special turn of the lip or 

 width of neck to appear at the right time, but because the mould 

 is the same in each case. 



The development of the individual is the building up of the 

 mould which forms the next generation (i.e., the germ cells), which 

 in most cases is all that is concerned, but in mammals at any rate 

 the moulding is continued to a much later period of the succeeding 

 generation's span of existence. 



The attraction and repulsion observed between cell and cell 

 are certain of the manifestations of this supposed form of energy 

 but probably not by any means all; just as attraction and 

 repulsion are manifestations of electrical energy under certain 

 conditions, but are not by any means the only manifestations. 

 In nerve impulses we may, for instance, really be experiencing 

 manifestations in another way of the same form of energy which 

 under other conditions produces the attractions and repulsions 

 and the figures of strain in the dividing cells, and the actual cell 

 division. 



Herbst made the remarkable observation that the segments of 

 developing Echinus eggs and others when allowed to develop in 

 sea water which has been deprived of calcium salts, separate from 

 one another, instead of adhering ; and that although they live so 

 long as to develop cilia as in a fully formed blastula, nevertheless 

 they do not form a completely closed vesicle. This observation 



