334 Report S.A.A. Advancement of Science. 



by the formation of fresh cell-combinations of the same kind as 

 existed in the original embryo. 



The phenomena of regeneration of lost parts in animals ami the 

 striking of roots by cuttings in the case of plants are only more 

 complex illustrations of the manner in Avhich a disturbance of the 

 equilibrium of an organism — whether between the constituent parts 

 of the organism itself or between the organism and its environment — 

 is remedied by re-arrangements which result in the old form being 

 again produced. We must suppose that the unbalanced forces re- 

 maining over after the disturbance act as stimuli, which evoke^ 

 structural modifications of whatever character ma\ be necessary 

 to restore the equilibrium. 



Any cell of the body is originally potentially a germ-cell, but the 

 great majority of the cells lose the power of de^•e]()ping into new 

 organisms in becoming specialised to meet the requirements of their 

 special environment, including under that term the surrounding cells 

 of the body. The extent to which this power may be presen'ed, 

 however, by ordinary somatic cells is very well shewn in the case 

 of the Begonia leaf, cells of which have the power of giving rise to 

 new and complete plants. 



It appears to be an invariable rule, however, at any rate in all 

 the higher animals, that special germ-cells are set apart at a very 

 early stage in the development of the organism for the sole purpose 

 of reproducing the species. These cells are especially distinguished 

 by their undifferentiated character. Whereas the somatic cells 

 become variously specialised in accordance with their varied func- 

 tions, some as muscle cells, others as nerve cells, and so on, the 

 germ-cells do not become specialised for any somatic function, but 

 are so situated as tO' be protected as much as possible from the in- 

 fluences of the environment, and thus hindered from specialisation. 

 There is no reason, however, to suppose that they are so far removed 

 from the influence of the somatic cells as tO' be prevented from 

 receiving and storing up stimuli which they may subsequently give* 

 out again under appropriate conditions. We have already seen reason 

 to believe that stimuli may be transmitted from cell to cell 

 without the aid of material particles and without the aid of proto- 

 plasmic connection, while the phenomena of after-effects have taught 

 us that stimuli may be stored up by living protoplasm for future use. 



In accordance wdth the principle of internal equilibration the 

 forces stored in the germ-cells must be in a state of equilibrium with 

 all the forces exerted by the body — the l)ody and the germ-cells must 

 balance one another. 



We may suppose that up to the time of their maturation the 

 germ-cells are in a receptive condition, capable of receiving and 

 storing stimuli from the surrounding soma. 



In the process of maturation we know that the germ-cells lui- 

 dergo very curious changes, and there is reason to- believe that in the 

 female cell at any rate a portion of the nucleus is cast out. The 

 ovum also commonly becomes in\ested in a \itelline membrane, 

 while the .spermatozoon acquires a vibratile tail. In other words. 



