THE AXIS OF THE EMBRYO. 151 



axis of the body located and pre-marked by the first plane of 

 cleavage, has steadily lost ground. 



The study of cleavage has not yet given us any such funda- 

 mental law of development as the mosaic theory claims. The 

 number of animals whose development does not conform to the 

 supposed law of coincidence has increased so rapidly of late, 

 that the only reasonable conclusion seems to be that while the 

 first cleavage plane may coincide with the median axis of the 

 embryo, as Roux and others have shown, it is not a constant 

 rule in any single case, much less a universal law. 



The opinion is gaining ground that the phenomena of cleav- 

 age are to be regarded as the expression of non-differential 

 rather than qualitative divisions of the germinal material. The 

 pressure experiments have proved that there can be extreme 

 variation in cleavage produced without in any way interfering 

 with the normal development of the embryo. 



NOTE. Batrachus tau = Opsanus tau. See Bulletin of U. S. National 

 Museum. No. 47, Part 3, p. 2313. 1898. 



