142 Organisms from Eggs 



if Roux's experiment is repeated with the modification 

 that the egg is put upside down after the destruction 

 of the one cell, the intact cell will give rise not to a half 

 but to a whole embryo.^ These experiments prove 

 that each of the first two cleavage cells of the frog's 

 egg represents one-half of the embryo and that a whole 

 embryo can develop from each half only when a re- 

 distribution of material takes place, which in the egg 

 of the frog can be brought about by gravitation since 

 the egg consists of a lighter and a heavier mass. 



When, therefore, in the egg of the sea urchin each 

 of the first two blastomeres naturally gives rise to a 

 whole embryo it is due to a greater degree of fluidity 

 of the protoplasm and not to a lack of preformation 

 of the embryo in the cytoplasm. This idea is confirmed 

 by the observations on the egg of Ctenophores whose 

 cytoplasm seems to be more solid than that of most 

 other eggs. Chun found that the isolated blastomere 

 of the first cell division produced a half -larva, possessing 

 only four instead of the eight locomotor plates of the 

 normal animal. 



It seems that in the egg of molluscs, also, the simple 

 symmetry relations of the body are already preformed. 

 It is well known that there are shells of snails which 

 turn to the right while others turn in the opposite 

 direction. The shells of LymncBus turn to the right, 

 those of Planorhis to the left. It was observed by 



* Morgan, T. H., Embryology of the Frog. New York. 



