200 PHYSIOLOGY CHAP. 



selves during the development through which the homunculus 

 becomes the homo sapiens. The most vulnerable point of this theory 

 is that in the superior organisms each individual is developed 

 by the co-operation of two generators of opposite sex. 



When in 1677 Leuwenhock described the spermatozoa, an 

 ardent discussion arose on the question whether the preformed 

 germ was represented by the ovum or the spermatozoon. The 

 dispute between the ovists and the spermatozoists lasted a century. 

 The former, amongst whom Spallanzani ranged himself, considered 

 that the ovum was the organism in miniature, and that the 

 spermatozoon had no other office than to excite it, and promote its 

 growth ; the latter, on the other hand, looking at the spermatozoon 

 with the microscope, claimed to distinguish in it besides the 

 head, also the arms, the bones, in fact the principal organs of the 

 adult, reduced to the lowest terms. For them the spermatozoon 

 was the organism in miniature, and the ovum was only the 

 nutritive medium necessary for its growth. 



Gaspar Frederick Wolff, in his thesis for the doctorate main- 

 tained in 1759 and published in 1764, performed the service of 

 substituting for the dogma of preformation the scientific principle 

 that there cannot be admitted as existent in the germ that which 

 the senses are incapable of perceiving. For him the germ is 

 primarily only a secretion of the genital organs of the generators, 

 without organisation in fact. It is in consequence of fertilisation 

 that it is organised gradually during development. 



According to this theory, which was called that of epigenesis, 

 the organs were differentiated one from the other at the expense 

 of the originally undifferentiated germinal substance. Guided 

 by observation Wolff traced the first stages of this process of 

 differentiation, and laid the foundation of the magnificent edifice 

 of the embryology of the last century. 



The theory of epigenesis, however, as it was conceived by 

 Wolff, and developed and perfected by embryologists after him, 

 met with great opposition. 



How was one to admit in fact that the natural forces 

 known to us should succeed in transforming in a few days, or 

 weeks, unorganised matter into an animal organism similar to 

 the generators ? An entirely new organisation from a sub- 

 stance quite unorganised resembles too much a creatio ex nihilo 

 to be admitted, for it is incapable of any scientific explanation 

 whatever. 



The nisus formativus imagined by Blumenbach, which impels 

 the unorganised paternal and maternal reproductive juices to 

 assume a determined form during development, to repair losses or 

 waste of organs, and sometimes even casual mutilations which 

 may occur to the adult organism, is nothing more than a vague 

 expression, which does not at all explain the unknown phenomenon, 



