i 3 o THEORIES OF HEREDITY 



that the pre-localization in normal development of each 

 half of the frog in one of the two first cells does not 

 warrant the conclusion that such pre-localization is 

 a fundamental or essential phenomenon. For, as shown 

 above, mere change of position will compel each of 

 these two cells to manifest a far wider potentiality 

 the potentiality of the whole body instead of one of its 

 halves. The most crucial test which could be applied 

 to Roux's conclusion would be not to injure but to 

 remove altogether one of the first two cells of the egg. 

 This is unfortunately impossible in the case of the frog ; 

 for the single remaining cell collapses. The experiment 

 has, however, been successfully performed upon the egg 

 of the newt (Triton), 1 by Herlitzka, who ' found that each 

 blastomere gives rise to a perfect, whole embryo of half- 

 size '. ' Thus we see ', Morgan concludes, ' that whatever 

 the factors may be that determine the development of 

 a single embryo from the egg, still each half, and perhaps 

 each fourth also, has the power of producing a whole 

 embryo.' 2 When we consider the development of widely 

 different animals we are led to widely different con- 

 clusions. Thus, according to Driesch, ^ of the egg 

 of an Echinoderm (Sea-urchin) can undergo at least 

 the preparatory stages of complete development. 



It has been shown that the planes of division by 

 which the egg is cut up into cells may bear no relation- 

 ship whatever to the position of the future embryo. 

 Dr. Jenkinson has even proved that the position of the 

 embryo frog, although predetermined in the egg itself, 

 is nevertheless without relationship to the direction of 

 the furrow which divides the egg into the two first cells. 3 



Returning to Diagram //, the second division pro- 

 duces the four cells of Stage 3, indicated by the letters 



1 Dr. J. W. Jenkinson points out that by a clerical error Professor 

 T. H. Morgan (I.e. p. 226} has spoken of the salamander (Salamandra) 

 instead of the newt (Triioti). Herlitzka's two papers (Arch./. Entivick.- 

 Mech. d. Organism., ii. 1896, p. 352; iv. 1897, p. 624 : Leipzig) describe 

 experiments upon Triton (Molge) cristatus. 



2 1. c. p. 226. 



3 Bwmeirika, vol. v, pts. i and ii, Oct. 1906, On the Relation between 

 the Symmetry of the Egg and the Symmetry of the Embryo in the Frog. 



