SEVENTH LECTURE. 



RELATION OF THE AXIS OF THE EMBRYO TO 

 THE FIRST CLEAVAGE PLANE. 



CORNELIA M. CLAPP, 



MT. HOLYOKE COLLEGE, SOUTH HADLEY, MASS. 



WHAT is the meaning of cleavage ? is the constantly recur- 

 ring question of the embryologist. Is cleavage a differentiating 

 process in development, or is it a process which may run on 

 independently of differentiation, sometimes coinciding with it, 

 sometimes not ? 



Is the egg a mosaic, as Roux maintains? Does the first 

 plane of cleavage determine the axial position of the embryo, 

 or is the egg already definitely oriented with respect to the 

 future embryo before cleavage begins ? Is the egg an isotropic 

 body to be converted gradually into a definite mosaic as cleav- 

 age advances, or are the blastomeres all endowed with the same 

 potentialities, the fate of each being settled by the position it 

 happens to hold ? 



Does the division of the egg by the first line of cleavage 

 mean the separation of the parts destined to become the right 

 and left sides of the bilaterally symmetrical animal, or may this 

 first cleavage run in any direction, and have no fixed and 

 necessary relation to the future embryo ? 



The well-known discoveries of Newport ('54) and of Pfluger 

 and Roux in '87 and '88 seem to point toward some general 

 law controlling the phenomena of cleavage. 



In 1891, while studying the development of Batrachus tan 

 at the Marine Biological Laboratory at Wood's Holl, Professor 

 Whitman directed my attention to the fact that the egg 



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