HEREDITY. 



71 



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 conditions. 



A fertilised egg cell with certain characters (a, b, c), de- 

 velops into an organism in which these characters are vari- 

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

 set apart, retaining the characters a, b, c, 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 " — (Sagitta, Thread-worms, Leeches, Polyzoa), 

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

 Chironomus among Insects, Phalangids among Spiders), in 

 Micrometres aggregates 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. But 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. 



Finally, it may be noticed in connection with heredity, 

 that there is great doubt to what extent the " body " can 

 definitely influence its own reproductive cells. Animals 

 acquire individual bodily peculiarities in the course of their 

 life, as the result of what they do or refrain from doing, or 

 as dints from external forces. The " body " is thus changed, 

 but there is much doubt whether the reproductive cells 

 within the " body " are affected by such changes. Weis- 

 mann denies the transmissibility of any characters except 



