HEREDITY. 73 



Rauber, Galton, Jager, Brooks, Nussbaum, and especially 

 emphasised by Weismann, is fundamentally important. 



At an early stage in the development of the embryo the 

 future reproductive cells of the organism are often dis- 

 tinguishable from those which are forming the body. These, 

 the somatic cells, develop in manifold variety, and, as 

 division of labour is established, they lose their likeness to 

 the fertilised ovum of which they are the descendants. The 

 future reproductive cells, on the other hand, are not impli- 

 cated in the formation of the " body," but, remaining 

 virtually unchanged, continue the protoplasmic tradition 

 unaltered, and are thus able to start an offspring which will 

 resemble the parent, because it is made of the same 

 protoplasmic material, and develops under similar con- 

 ditions. 



A fertilised egg-cell with characters (a, />, c . . . x, y, z), 

 develops into an organism in which these characters are 

 expressed; but if, at an early stage, certain cells are set 

 apart, retaining the characters a, & 9 c . . . .T, y, z, in all 

 their entirety, then each of these cells will be on the same 

 footing as the original fertilised egg-cell, able to give rise to 

 an organism, almost necessarily to a similar organism. 



An early isolation of reproductive cells, directly con- 

 tinuous and therefore presumably identical with the original 

 ovum, has been observed in the development of some 

 " worm types " (Sagitfa, Thread-worms, Leeches, Polyzoa), 

 and of some Arthropods (e.g. Moina among Crustaceans, 

 Chironomus among Insects, Phalangidse among Spiders), in 

 Micrometrus aggregatus among Teleostean fishes, and with 

 less distinctness in some other animals. 



In many cases, however, the reproductive cells are not 

 recognisable until a relatively late stage in development, 

 after differentiation has made considerable progress. Weis- 

 mann gets over this difficulty by supposing that the con- 

 tinuity is sustained by a specific nuclear substance the 

 germ-plasm -- which remains unaltered in spite of the 

 differentiation in the body. It is perhaps enough to say 

 that, as all the cells are descendants of the fertilised ovum, 

 the reproductive cells are those which retain intact the 

 qualities of that fertilised ovum, and that this is the reason 

 why they are able to develop into offspring like the parent. 



