PKE-FORMATION AND EPIGENESIS 163 



parents. Everyone is familiar with cases of this kind. It may 

 he some abnormality of fingers or toes, or a lock of white hair in 

 some special situation in a dark-haired man, or even some 

 trifling nervous habit, that is thus indelibly impressed upon the 

 organism and handed on from one generation to another. 



Inasmuch as the only possible connection between parent and 

 offspring is (in most cases) through the germ cells, it follows that 

 there must be something in these germ cells which, so to speak, 

 represents all the inheritable characters of the parents and is 

 capable of giving rise to a repetition of these characters in the 

 course of individual development. 



Two sharply contrasted views as to what takes place in the 

 development of the egg were maintained by the older embryo- 

 logists, and, in a modified form, survive to the present day. The 

 so-called " evolutionists," or " pre-formationists," maintained 

 that the egg contains in itself a complete miniature of the 

 organism into which it develops, and that tbe process of 

 development consists simply in an unfolding (" evolution ") and 

 growth of this miniature. This idea, of course, carried to its 

 logical conclusion, involves the further supposition that every 

 egg contains in miniature the bodies of all future generations, 

 like nests of boxes one within the other. 



In opposition to this view the upholders of the theory of 

 " epigenesis " maintained that there is no pro-formation of organs 

 in the egg but that the different parts of the adult organism 

 become gradually differentiated from the simple undifferentiated 

 ovum during the course of development. This view, which is 

 said to have originated with Aristotle and was strongly supported 

 by the great pioneer embryologist C. F. Wolff about the middle 

 of the eighteenth century, almost entirely superseded the crude 

 ideas of the pre-formationists, but at the present day the latter 

 are being revived to some extent, but in a more refined form, as a 

 result of modern experiments in embryology. It cannot be 

 disputed that in some cases certain parts of the adult organism 

 can be traced back to corresponding portions of the egg, which 

 cannot therefore be entirely undifferentiated, and it is probable 

 that in the end the truth will be found, as in so many other 

 cases, to lie in a compromise between the two extreme views. 



The great problem which has to be solved by any theory of 

 heredity is How do the apparently simple germ cells of a multi- 

 cellular organism come to be representative of all the other 



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