HISTORICAL SKETCH 425 



published showing a small human figure, or "homunculus," within the 

 sperm head. The ovists, on the contrary, held that the individual is 

 encased in the egg. A bitter strife was carried on over this question by 

 the two groups and various interesting compromises were made, but all 

 extreme forms of preformationism were to disappear in the light of more 

 critical investigations, which went far to support the opposing theory of 

 epigenesis. 



Two of the early champions of the theory of epigenesis were William 

 Harvey (1578-1667; Exercitationes de Generatione Animalium, 1651), and 

 Caspar Friedrich Wolff (1733-1794; Theoria Generationis, 1759). As the 

 result of many careful observations on the embryogeny of the chick, 

 Wolff was able to show beyond question that development is epigenetic: 

 neither egg nor spermatozoon contains a formed embryo; development 

 consists not in a process of unfolding, but in "the continual formation of 

 new parts previously non-existent as such" (Wilson). Here was room 

 for the principle of true generation, or "the production of heterogeneity 

 out of homogeneity." Wolff also discovered the vegetative growing- 

 point of plants and described the new formation of the successive lateral 

 members. The Theoria Generationis is to be regarded as one of the really 

 great contributions to biological science, for the theory of epigenesis, to 

 which it furnished substantial support, later became established with 

 modifications as a fundamental principle of embryology, particularly 

 through the work of Karl Ernst von Baer (1792-1876) in the nineteenth 

 century. 



In commenting on preformation and epigenesis Whitman (1894) 

 emphasizes the fact that the tendency of modern biology has not been to 

 show the entire falsity of either of these views, but to seek out the germs of 

 truth possessed by each, and to relate them to modern biological concep- 

 tions. Our present position, although it excludes both views in their 

 crude original form, involves in a new sense both conceptions. When we 

 say that the egg is organized, possessing an architecture or mechanism 

 in its cytoplasm or nucleus which largely predetermines the course of 

 development, we are making a modernized statement of the preformation 

 idea. When we say that the parts of the individual are in no way 

 delineated in the egg but are mainly determined by external conditions 

 during the course of development, we are speaking in terms of modern 

 epigenesis. "The question is no longer whether all is preformation or all 

 postformation; it is rather this: How far is postformation to be explained 

 as the result of preformation, and how far as the result of external influ- 

 ences?" (Whitman). When, therefore, it is borne in mind that one of the 

 outstanding problems of modern cytology is that of identifying the factors 

 involved in the development of an organized and highly differentiated 

 individual from an organized but relatively undifferentiated egg, it is 

 evident that any sketch of cytological history would be incomplete 



