THE THEORY OF GERMINAL LOCALIZATION 



ming who gave the first specific statement of the matter from the cyto- 

 logical point of view : " But if the substance of the egg-cell has a 

 definite structure (Bau), and if this structure and the nature of the 

 network varies in different regions of the cell-body, we may seek in 

 it a basis for the predetermination of development wherein one egg 

 differs from another, and it will be possible to look for it with the 

 microscope. How far this search can be carried no one can say, but 

 its ultimate aim is nothing less than a true morphology of inheritance" 1 

 In the following year Van Beneden pointed out how nearly this con- 

 ception approaches to a theory of preformation : " If this were the 

 case (i.e. if the egg-axis coincided with the principal axis of the adult 

 body), the old theory of evolution would not be as baseless as we 

 think to-day. The fact that in the ascidians, and probably in other 

 bilateral animals, the median plane of the body of the future ani- 

 mal is marked out from the beginning of cleavage, fully justifies the 

 hypothesis that the materials destined to form the right side of the 

 body are situated in one of the lateral hemispheres of the egg, while 

 the left hemisphere gives rise to all of the organs of the left half." 2 



The hypothesis thus suggested seemed, for a time, to be placed on 

 a secure basis of fact through a remarkable experiment subsequently 

 performed by Roux ('88) on the frog's egg. On killing one of the 

 blastomeres of the two-cell stage by means of a heated needle the un- 

 injured half developed in some cases into a well-formed half -larva 

 (Fig. 182), representing approximately the right or left half of the 

 body, containing one medullary fold, one auditory pit, etc. 3 Analo- 

 gous, though less complete, results were obtained by operating with 

 the four-cell stage. Roux was thus led to the declaration (made with 

 certain subsequent reservations) that " the development of the frog- 

 gastrula and of the embryo formed from it is from the second cleavage 

 onward a mosaic-work, consisting of at least four vertical indepen- 

 dently developing pieces." 4 This conclusion seemed to form a very 

 strong support to His's theory of germinal localization, though, as 

 will appear beyond, Roux transferred this theory to the nucleus, and 

 thus developed it in a very different direction from Lankester or 

 Van Beneden. His's theory also received very strong apparent sup- 

 port through investigations on cell-lineage by Whitman, Rabl, and 



1 Zellsubstanz, '82, p. 70 : the italics are in the original. 



2 '83, P. 571. 



3 The accuracy of this result was disputed by Oscar Hertwig ('93, i), who found that the 

 uninjured blastomere gave rise to a defective larva, in which certain parts were missing, but 

 not to a true half-body. Later observers, especially Schultze, Endres, and Morgan, have, 

 however, shown that both Hertwig and Roux were right, proving that the uninjured blasto- 

 mere may give rise to a true half-larva, to a larva with irregular defects, or to a whole 

 larva of half-size, according to circumstances (p. 422). 



4 I.e., p. 30. 



