GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 



369 



required to show that this purely mechanical factor, though doubtless 

 real, must be subordinate to some other. This is strikingly shown, 

 for example, in the development of annelids and mollusks, where the 

 spiral cleavage, strictly maintained during the earlier stages, finally 

 gives way more or less completely to a bilateral type of division in 

 which the rule of minimal surface-contact is often violated. We see 

 here a tendency operating directly against, and finally overcoming, 



Fig. 171. Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning 

 and of a marked determinate character. 



A. Two-cell stage (the circles are oil-drops). B. Four-cell stage ; the second cleavage-plane 

 passes through the future median plane. C. The same from the right side. D. Eight-cell stage. 

 E. Sixteen cells; from the cells marked /arises the prototroch or larval ciliated belt, from X the 

 ventral nerve-cord and other structures, from D the mesoblast-bands, the germ-cells, and a part of 

 the alimentary canal. F. Twenty-nine-cell stage, from the right side ; /. girdle of prototrochal cells 

 which give rise to the ciliated belt. 



the mechanical factor which predominates in the earlier stages ; and 

 in some cases, e.g. in the egg of Clavelina (Fig. 177) and other tuni- 

 cates, this tendency predominates from the beginning. In both 

 these cases this " tendency " is obviously related to the growth-process 

 to which the future bilateral embryo will owe its form ; 1 and every 

 attempt to explain the position of the cells and the direction of cleav- 

 age must reckon with the morphogenic process taken as a whole. 

 The blastomere is not merely a cell dividing under the stress of rude 



1 Cf. Wilson ('92, p. 444). 



2B 



