Cii. IV] CLEAVAGE OF THE EGG 47 



four blastomeres, the inner end of the reduced blastomere 

 retained its central position. In another instance material was 

 taken out of that one of the four blastomeres that had alread}- 

 made a broad cross-line with its vis-d-vis. Although this 

 blastomere was much reduced in size and made smaller than 

 any other blastomere of the system, yet it retained the same 

 cross-line as before ; i.e. it was not pushed out to the periph- 

 ery. Even when the experiment was made at the time of 

 appearance of the second cleavage, the newly fornung blasto- 

 meres did not in all cases adjust themselves in agreement with 

 the laws regulating the oil-drops. 



These results show that the conditions present in the frog's 

 eo-e do not allow the blastomeres to assume always the arrange- 

 ment shown by the same number of oil-drops having the same 

 relative size. Roux points out several differences in the two 

 cases. The walls of neighboring blastomeres seem to stick 

 together, and this would prevent the blastomeres from gliding 

 freely over one another should any change take place to disturb 

 the equilibrium. Moreover, the blastomeres are living con- 

 tractile bodies, and through their own internal activity may 

 interfere with the mechanical tendencies of the system. The 

 nature of the surface of each blastomere and the sort of changes 

 taking place in the surface may also affect the arrangement. 



It will be seen then, as has been said, that there may be fac- 

 tors present in the frog's egg that so influence the arrangement 

 of the blastomeres that the systems do not always conform to 

 those of the oil-drops. Nevertheless, the results from the latter 

 give us an ideal scheme showing the effect produced by one set 

 of factors, — that of surface-tension. It seems highly probable 

 that surface-tension is also an important factor in the segment- 

 ing egg, but other conditions present prevent its free play. 



Historical Account of the Cleavage of the Frog's 



Egg 



The earliest observations on the segmentation of the animal- 

 ovum were made upon the frog's egg. Swammerdam ('37) 

 saw, but did not understand, the first cleavage-furrow of the 

 egg. Spallanzani, in 1785, observed the first two furrows cross- 



