298 THEORIES OF INHERITANCE AND DEVELOPMENT 



and every step in the process bears a definite and invariable relation 

 to antecedent and subsequent steps. ... It is, therefore, not sur- 

 prising to find certain important histological differentiations and 

 fundamental structural relations anticipated in the early phases of 

 cleavage, and foreshadowed even before cleavage begins." ^ It was, 

 however, Flemming who gave the first specific statement of the 

 matter from the cytological point of view : " But if the substance of 

 the egg-cell has a definite structure (Ban), 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 develop- 

 ment wherein one egg differs from another, and it will be possible to 

 look for it zvith tJie viicroscopc. How far this search can be carried 

 no one can say, but its ultimate aim is nothing less than a true 

 viorpliology of inheritance? In the following year Van Beneden 

 pointed out how nearly this conception 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 animal 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 Qg^^, while the left hemisphere gives rise to all of 

 the organs of the left half." '^ 



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 

 uninjured half developed in some cases into a perfectly formed half- 

 larva (Fig. 131), accurately representing the right or left half of the 

 body, containing one medullary fold, one auditory pit, etc.* 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 



1 '78, p. 49- 



"■2 Zellsubstanz, '82, p. 70; the italics are in the ongmal. 



^ '83, p. 571. 



4 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 perfect half-larva, to a larva with irregular defects, or to a whole 

 larva of lialf-size, according to circumstances (p. 319). 



