60 



CLEAVAGE 



II 



This theory was founded upon an observation and an experi- 

 ment made by R.OUX upon the egg of the Frog. Roux believed 

 that observation told him that the first furrow of the egg 

 invariably coincided with the median longitudinal plane of 

 the embryo, the second with the transverse plane (putting 

 those cases aside in which the first furrow by an ' anachronism ' 

 occupied the position of the second). Hence of the two blasto- 

 meres one contained the determinants for the right half, the 



7TI 



-17L 



FIG. 12. A and B. Normal Frog embryos with medullary folds (m), 

 open (^4) and closed (B}. C. Hemiembryo dexter with almost complete 

 post-generation of the ectoderm ; u, yolk-plug. D. The same, older, but 

 with less post-generation. E. Hemiembryo anterior (?) with beginning 

 post-generation. (From Korschelt and Heider, after Roux.) 



other those for the left half of the body. This was confirmed 

 by experiment, for when one of the blastomeres was killed by 

 means of a hot needle the survivor was found to give rise to a 

 half-embryo, right or left as the case might be (Fig. 12). Later 

 investigation has, however, been able wholly to confirm neither 

 the observation nor the experiment, while the institution of 

 experiments on the ova not only of the Frog but of other 

 animals has shown that a cell-division is not the cause of 



