izo SEGMENTATION [CH. 



the egg undergo re-arrangement immediately after the en- 

 trance of the spermatozoon in such a way as to give the 

 egg a bilateral symmetry, and the meridian of symmetry 

 always coincides with the subsequent sagittal plane of 

 the embryo, whether the first cleavage plane does so or 

 not. Roux showed that although the first cleavage plane 

 of the Frog usually includes the entrance path of the 

 spermatozoon it is easy to alter this arrangement by 

 compressing the egg in a glass tube, in accordance with 

 the law of cytoplasmic division mentioned in Chapter III, 

 and presumably in the relatively small proportion of 

 naturally developing eggs in which the two planes do not 

 coincide, the re-arrangement of the materials of the egg 

 consequent on the entrance of the spermatozoon takes place 

 in such a way as to involve an alteration in the position of 

 the first cleavage spindle similar to that induced by com- 

 pression. 



In the second and later segmentation divisions the ar- 

 rangement of the cleavage planes depends on the law just 

 referred to that the spindle takes up its position in the 

 longest axis of the cell-protoplasm as distinct from yolk. 

 In eggs in which there are materials of different sorts, these 

 become segregated sooner or later into separate blastomeres 

 or groups of blastomeres, and it is pointed out by BRACKET 

 that during segmentation there is no true cell-differentia- 

 tion, in the sense of the production of new structures in 

 some of the cells, but only Cc a cutting up of the egg into 

 smaller and smaller territories 1 " which differ from one 

 another in including different parts of the original egg 

 materials. The relation of this segregation of the egg 

 materials into separate blastomeres to true cell-differentia- 

 tion will be referred to in a later chapter. Apart from its 

 1 BRACKET, 1917, p. 259. 



