86 EXTERNAL FACTORS III. i 



the plane of the wheel, is constant, a third set were packed 

 loosely in test-tubes, and so able to roll over one another in all 

 directions as they fell from one end of the tube to the other 

 with each revolution. The first furrow appeared in all these 

 eggs at the normal time and it was meridional, as in the normal 

 egg ; similarly the second was meridional, the third latitudinal ; 

 but the egg-axes exhibited no definite relation either to the 

 vertical or to the plane of the wheel. The eggs were allowed 

 to continue their development on the apparatus, and gave rise 

 to normal tadpoles. 



From this experiment Roux drew the conclusion that it is not 

 gravity which determines the direction of the planes of cleavage, 

 and that gravity is not an indispensable necessity for the normal 

 development of the egg of the Frog. 



Incontrovertible though this conclusion appears to be on the 

 evidence, it has nevertheless been disputed by certain embryo- 

 logists, Schulze and Moszcowski, the controversy between whom 

 and Roux upon the subject has now extended over many years. 



Schulze has urged (1) that eggs placed on such a machine 

 do not develop normally, and (2) that the rotation of the eggs 

 in their jelly exactly compensates for the rotation of the wheel. 

 With regard to the latter point Roux has replied that on this 

 supposition the egg-axes ought to be, at any moment, vertical, 

 which is not the case. To the first objection it is a sufficient 

 answer that not only Roux, but subsequent investigators 

 (Morgan, Kathariner and the present writer) have been able 

 to produce normal tadpoles from such rotated eggs. 



It may be noticed here that Kathariner has repeated Roux's 

 experiment with a slight variation. The eggs were kept con- 

 stantly rotating, not in one but in an indefinite number of 

 planes by a stream of air-bubbles passing through a glass vessel 

 filled with water. Development was normal. This result does not 

 differ materially from that obtained by Roux with the test-tube 

 eggs refei'red to above, which has indeed been also independently 

 corroborated by Morgan. 



The criticisms of Moszcowski take a different form. This 

 author urges that gravity always exercises an influence upon 

 the egg in determining the bilateral symmetry of both egg and 



