66 REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



variously 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 sanie footing 

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

 organism, almost necessarily to a similar organism. 



An early appearance or insulation of reproductive cells, 

 directly continuous and therefore presumably identical with 

 the original ovum, has been observed in the development 

 of some "worm-types" {Sagiita, Threadworms, Leeches, 

 Polyzoa), and of some Arthropods {e.g., Moina among 

 Crustaceans, Chironotnus among Insects, Phalangidse among 

 Spiders), and with less distinctness in some other animals. 



I need not explain the hypothesis by which Weisniann 

 gets over the difficulty that the reproductive cells sometimes 

 appear relatively late in development, after differentiation has 

 made considerable progress. It is enough to say that all 

 the cells are descendants of the fertilised ovum, that the 

 reproductive cells are those which retain intact the qualities 

 of that fertilised ovum, 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. Weismann 

 denies the transmissibility of any characters except those 

 inherent or congenital in the fertilised egg-cell, and therefore 

 denies that the influences of function and environment are, 

 or have been, of any importance in the evolution of many- 

 celled animals. Such influences affect the body, but do not 

 reach its reproductive cells, and are therefore non-trans- 

 missible. Many of the most authoritative biologists are at 

 present of this opinion. On the other hand, many still 

 maintain, as I do, that profound changes due to function or 

 environment may saturate through the organism, and affect 

 the reproductive cells, and thus the race. But the whole 

 question is under discussion. (See Article Heredity, in 

 the new edition of Chambers's Encydopxdia.) 



