Lefevre — The Advance of Zoology in the Nineteenth Century. 101 



inated from the sexual organs of the parent, which gradually 

 becomes organized, but only during the process of develop- 

 ment in consequence of fertilization." According to Wolff 

 therefore the organs of the body are only gradually differen- 

 tiated during development out of an originally undifferen- 

 tiated germinal material. For many years his theory was 

 buried in obscurity, but it was later brought to light and 

 to-day he is accepted as the founder of the theory of epi- 

 genesis, the rival of Weismann's doctrine of preformation. 

 In recent times with the development of the cell-theory, with 

 a closer insight into the nature of cell-processes, and espe- 

 cially with the advance of our knowledge of the finer struc- 

 ture of the germ-cells, much in Wolff's doctrine of unorganized 

 germinal matter has had to be discarded, but the essential 

 conception of his theory of development has laid the founda- 

 tion of modern epigenesis. 



The two opposing points of view, preformation and epi- 

 genesis, around which the earlier discussion took place, 

 strange to say, furnish the modern contention in discussions 

 regarding the nature of development. Our more accurate 

 instruments and more refined methods, it is true, have forced 

 the abandonment of what was crude and grotesque in the 

 earlier views, but the fundamental conception of each theory 

 is the same and has been actively fought over in the last 

 decade. Although in recent preformation-theories it is not 

 maintained that the embryo is actually preformed in the germ 

 in its complete and final organization, the view has been 

 strongly advocated that the organism is predetermined in the 

 sense that different regions of the germ contain different sub- 

 stances which are destined to form definite parts of the 

 embryo ; in other words, that the head, for example, is formed 

 in development from a certain definite and predetermined 

 portion of the germinal substance and from that alone. 



Time will permit of but the merest reference to the final 

 product of these opposing theories, namely the modern Pre- 

 formation Doctrine, first formulated by Wilhelm Roux in 

 1883, but greatly elaborated and extended by Weismann, and 

 modern Epigenesis, whose chief exponent has been Oscar 

 Hertwig. 



