88 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



(4) Organic continuity between generations. — Heredity. — 

 Everyone knows that like tends to beget like, that offspring 

 resemble their parents and their ancestors. Not only 

 are the general characteristics reproduced, but minute 

 features, idiosyncrasies, and pathological conditions, inborn 

 in the parents, may recur in the offspring. 



At an early stage in the development of the embryo the 



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Fig. 42. — Larvae of Common EeL (Not drawn to scale.) 



I. Smallest l-nown larva, 7 mm. in length. II. and III. Laterally compressed 

 transparent Leptocephalus stages. I\'. Change, when about two years old, from 

 knife-blade-like to cylindrical shape. \'. Young elver, about three inches long, 

 about two and a half years old. 



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 

 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 



