4 INTRODUCTION TO CYTOLOGY 



held that the individual is encased in the egg. A bitter strife was carried 

 on over this question by the two groups of preformationists, and various 

 interesting compromises were made. But all extreme forms of preforma- 

 tionism 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 AnimaUum, 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 there was room for the principle of true generation, or "the 

 production of heterogeneity out of homogeneity." The Theoria Genera- 

 tionis 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 

 von Baer 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 or both of these views, but to seek out the 

 germs of truth possessed by each, and to relate them to modern biological 

 conceptions. "The two views missed the mark by over-shots in contrary 

 directions," says Whitman. The one theory claimed too much preforma- 

 tion: everything was preformed at the start. The other theory claimed 

 too much postf ormation : everything was formed anew. Our present 

 position, although it excludes both views in their crude origina-l 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 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 postf ormation; it is rather this : 

 How far is post-formation to be explained as the result of pre-f ormation, and 

 how far as the result of external influences? " When it is borne in mind, 

 therefore, that one of the outstanding problems of modern cytology is 

 that of identifying the factors involved in the development of an organ- 

 ized and highly differentiated individual from an organized but relatively 

 undifferentiated egg cell, it is at once evident that our sketch of cyto- 

 logical history would be incomplete without the above reference to the 

 early Theories of Preformation and Epigenesis. 



