FOURTEENTH LECTURE. 



THE SIGNIFICANCE OF THE SPIRAL TYPE OF 



CLEAVAGE AND ITS RELATION TO THE 



PROCESS OF DIFFERENTIATION. 



C. M. CHILD. 



During the sixty years since the cell theory was propounded 

 by Schleiden and Schwann (1838-39) it has come to dominate 

 almost completely the various departments of biological re- 

 search. In the introduction to his book, TJie Cell ('97), Wilson 

 remarks that " it has become even more clearly apparent that 

 the key to all ultimate biological problems must, in the last 

 analysis, be sought in the cell." Notwithstanding the wide 

 acceptance of the cell theory, a voice of protest has from time 

 to time been heard, from both the botanical and the zoological 

 side, based upon actual observation as well as upon theoretical 

 considerations. Prominent among those who have upheld 

 what Whitman ('93) designates as the "organism standpoint" 

 are Sachs, Rauber, Adam Sedgwick, and Whitman himself. 

 It is not my intention to enter into a historical discussion of 

 the views of the various authors, but to direct attention to one 

 phase of the problem, viz., the relation of the process known 

 as differentiation or organization of the cell. 



As is universally recognized, differentiation may occur within 

 the limits of a single cell, as in the Protozoa, or within an 

 organism consisting of many cells, and, therefore, cannot 

 necessarily be connected with the process of cell formation. 

 This fact is most clearly expressed by Whitman as follows : 

 " It is not division of labor and mutual dependence that con- 

 trol the union of the blastomeres. It is neither functional 



231 



