376 



CELL-DIVISION AND DEVELOPMENT 



the distribution of metaplasmic materials is sufficient to explain the 

 position of the spindle, whether with reference to the direction or the 

 inequality of the cleavage. 



As regards the direction of the spindle, Berthold ('86) long since 

 clearly pointed out that prismatic or cylindrical vegetable cells, for 

 instance, those of the cambium, often divide lengthwise ; and numer- 

 ous contradictions of Hertwig's " law " have since been observed by 

 students of cell-Hneage with such accuracy that all attempts to explain 

 them away have failed.^ In some of these cases the position of the 

 spindle is not that of least but of greatest resistance,^ the spindle ac- 



Fig. 176. — Segmenting eggs of ^jcarw. [KOSTANECKI and SlEDLECKl.] 

 A. Early prophase of second division, showing double centrosomes. B. Second cleavage in 

 progress ; upper blastomere dividing parallel to long axis of the cell. 



tually pushing away the adjoining cell to make way for itself. Simi- 

 lar difficulties, some of which have been already considered (p. 372), 

 stand in the way of the attempt to explain the eccentricity of the 

 spindle in unequal division. All these considerations drive us to the 

 view that the simpler mechanical factors, such as pressure, form, and 

 the like, are subordinate to far more subtle and complex operations 

 involved in the general development of the organism, a conclusion 

 strikingly illustrated by the phenomena of teloblastic division (p. 371), 

 where the constant succession of unequal divisions, always in the 



1 Cf. Watase ('91), Mead ('94, '97, 2), Heidenhain ('95), Wheeler ('95), Castle ('96), 

 Jennings ('97). 



2 See especially the case observed by Mead ('94, '97, 2), in the egg oi Amphitrite. 



