368 THE WORLD OF LIFE 



"Even the two first clanghter-cells (E) which result from the 

 division of the egg-cell give rise in many animals to totally different 

 parts. One of them, by continued cell-division, forms the outer 

 germinal layer, and eventually all the organs which arise from it, 

 e. g. the epidermis, central nervous system, and sensory cells ; the 

 other gives rise to the inner germinal layer and the organs derived 

 from it — the alimentary system, certain glands, etc. The conclu- 

 sion is inevitable that the chromatin determining these hereditary 

 tendencies is different in the very first two daughter-cells." 



Later on he shows in great detail how^ similar but even 

 more complex changes take place in the newly fertilised germ- 

 cell in v^hich the male and female elements are combined, for 

 the purpose of bringing about the accurate partition of these 

 elements in all the cells which arise from them by subdivision, 

 thus rendering possible the production, in all future genera- 

 tions, of males and females in nearly equal proportions. He 

 also shows that there is a special provision for the produc- 

 tion of slight variations in successive generations in a way too 

 complex to be explained here. This, of course, is largely 

 speculation, but it is based at every step on observed facts 

 in the processes of fertilisation and cell-division.^ 



In Professor J. Arthur Thomson's most valuable and il- 

 luminating work on Heredity, in which he impartially ex- 

 pounds the theories and discoveries of all the great physio- 

 logical writers of the world, he gives a very high, if not the 

 highest, place to those of Weismann. I will therefore quote 

 from his volume Weismann's latest short statement of his 

 hypothesis as to the nature of the germ-plasm ; and also Pro- 



1 The reader will see that the diagrams referred to in Weisraann's state- 

 ments, quoted above, do not seem to represent accurately what he says. 

 They must, therefore, be taken as " diagrams " only, not detailed " figures " 

 of what is seen, which are often so complex that it is difficult to follow 

 the essential details. They are for the purpose of indicating definite stages 

 in the process of the development of cells up to the first cell-division. 

 The small letters (jd) are not referred to in Weismann's explanation on 

 the plate itself, nor in his description of what happens. But these letters 

 evidently mean " idants," as explained in Professor J. A. Thomson's sum- 

 mary of Weismann's theory at p. 20. 



