244 



BIOLOGICAL LECTURES. 



lished, however, as we find it, there is little reason to suppose 

 that, since its establishment, the blastomeres have lost their 

 connection physiological or otherwise with each other, 

 unless the existence of a high degree of self-differentiation 

 is actually proven. 



Now the spiral cleavage is, as shown above, the most favor- 

 able form of cleavage possible for the establishment of physical 

 continuity of organization, for each cell is brought into contact 

 with the greatest possible number of cells. Is it not a justifi- 

 able conclusion that its wide occurrence, firm establishment, and 

 invariability, especially in those forms in which a condensation 



FIG. 7. 



FIG. 8. 



FIG. 7. 1 Arenicola. Formation of cross by radial divisions. 



FIG. 8. A renicola. The terminal cells of the ventral arms of the " cross " show spindles which 

 are distinctly spiral. 



of development a precocious segregation is found, are due, 

 at least in some measure, to the fact that it affords the least 

 possible physical obstacle to intercommunication between the 

 component elements ? For, if a tendency to condensation be 

 present, the possession or acquisition of the spiral form of 

 cleavage must be of great advantage. Or it may be that the 

 presence of this form of cleavage has led to that form of con- 

 densation known as precocious segregation. Whichever alter- 



1 In this and following figures of cleaving eggs the annelid cross (" rosette- 

 series " of Conklin) and its descendants, and the somatic plate arising from 2d 

 are enclosed by a heavy line; and the cells of the prototroch and paratroch, so far 

 as formed in each case, are stippled. 



