346 EDWIN G. CONKLIN 



differentiation of the spongioplasm proceeds at least for a time, 

 but if the mitoses are stopped the differentiation of the spongio- 

 plasm is also stopped. 



There is no indication that the nucleus is undergoing differen- 

 tiation during cleavage; the nucleus in a micromere is evidently 

 of the same character as its sister in the macromere, as is proved 

 by those cases in which the third cleavage spindles are forced 

 into an equatorial position, thus giving rise to eight macromeres; 

 the daughter cells in this case are all macromeres in that each 

 gives off three micromeres, and the nucleus which would have 

 gone into a micromere lender normal conditions now goes into a 

 macromere without in any way changing its future differentia- 

 tions. In short the nuclei of macromeres and micromeres are not 

 differentiated at the time of their formation, but may be thrown 

 about, as Driesch has said, *like balls in a pile' without changing 

 the fate of any of the cells into which they go. 



It is thus possible to show that the differentiation of a blasto- 

 mere does not depend upon the differentiation of its nucleus nor 

 does it depend entirely upon the segregation of cytoplasm and 

 yolk, nor upon the direction or position of the cleavage plane. 

 After differentiation and localization of cytoplasmic substances 

 has already occurred the direction of cleavage is an important 

 factor, but not before. 



IV. GENERAL CONCLUSIONS 



1. The nature and causes of cell polarity 



Polar differentiation, or more briefly polarity, may be defined 

 as the condition of having unlike poles, particularly in the chief 

 axis of a body, while symmetry is the condition of having like 

 poles in certain axes. The polarity of an entire organism, or of 

 any of its parts, is an expression of the relative positions with 

 respect to the chief axis of subordinate parts having different 

 structures and functions; in this sense it is customary to speak 

 of the polarity of organisms, organs, cells, nuclei, etc., but there 

 is no evidence that the polarity of the entire organism is the re- 

 sultant of the polarities of its constituent parts, as is the case 

 with magnetic polarity. 



