72 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



that the reproductive cells, which give rise to the offspring, 

 are more or less directly continuous with those which gave 

 rise to the parent. This idea, suggested by Owen, Haeckel, 

 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 distinguishable 

 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 implicated 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 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 insulation 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, Threadworms, Leeches, Polyzoa), 

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

 Chironomus among Insects, Phalangidae 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 differen- 

 tiation 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 



