372 



CELL-DIVISION AND DEVELOPMENT 



firmed by O. Hertwig ('98), who, by placing frogs' eggs in a centrifu- 

 gal machine, has caused them to undergo a meroblastic cleavage 

 through the artificial accumulation of yolk at the lower pole, due to 

 the centrifugal force. 



While doubtless containing an element of truth, this explanation is, 

 however, no more adequate than Balfour's rule regarding the relation 

 between deutoplasm and rhythm (p. 366) ; for innumerable cases are 

 known in which no correlation can be made out between the distribu- 

 tion of inert substance and the inequality of division. This is the 

 case, for example, with the teloblasts mentioned above, which contain 

 no deutoplasm, yet regularly divide unequally. It seems to be inap- 



r- / 



Fig. 173. — Partial or meroblastic cleavage in the squid Loligo. [Watase.] 



plicable to the inequalities of the first two divisions in annelids and 

 gasteropods. It is conspicuously inadequate in the history of indi- 

 vidual blastomeres, where the history of division has been accurately 

 determined. In Nereis, for example, a large cell known as the first 

 somatoblast, formed at the fourth cleavage (Jf, Fig. 171, E), under- 

 goes an invariable order of division, three unequal divisions being fol- 

 lowed by an equal one, then by three other unequal divisions, and 

 again by an equal. This cell contains little or no deutoplasm and 

 undergoes no perceptible changes of substance. 



The collapse of the rule is most complete in case of the rudi- 

 mentary cells referred to above. In some of the annelids, e.g. in 

 Aricia, where they were first observed,^ these cells are derived from 

 the very large primary mesoblast-cell, which first divides into equal 

 halves. Each of these then buds forth a cell so small as to be no 

 larger than a polar body, and then immediately proceeds to give rise 



1 Cf. Wilson, '92, '98. 



