254 SOME RECENT 



The second cleft is usually at right angles to the 

 first, and divides the egg into anterior and posterior 

 halves; but it may cut the egg obliquely. The 

 third and fourth clefts also present considerable 

 variability in their position and relations to the 

 earlier clefts. In all the cases described above the 

 variability was confined to the earliest phases of 

 segmentation : the earlier stages of development 

 were the same, whatever the mode in which seg- 

 mentation was effected ; and apparently identical, 

 and certainly normal embryos, resulted in all 

 cases. 



Some recent observations of Loeb give a possible 

 clue to these phenomena. He found by experi- 

 menting on the eggs of sea urchins that the process 

 of segmentation could be retarded, or modified, by 

 varying the proportion of sodium chloride present 

 in the sea- water in which the eggs were laid. 

 A slight increase in the normal proportion of sodium 

 chloride delayed the occurrence of segmentation ; 

 but on the return of the eggs to normal sea-water 

 they very quickly divided, often simultaneously, 

 into a number of blastomeres. He states his results 

 thus : " If we bring impregnated eggs into sea- 

 water of a certain higher concentration, no segment- 

 ation takes place ; but if we bring them back into 

 normal sea-water, they divide in about twenty 

 minutes directly into nearly, but not quite so many, 

 cleavage spheres as they would contain by that time 

 if they had remained in normal sea-water all the 

 time." Further investigation showed that, although 

 when placed in water containing more than the 



