GEOMETRICAL RELATIOXS OF CLEAVAGE-FORMS 2/3 



in the statement ('80) that the size of the cells formed in cleavage 

 varies inversely to the relative amount of protoplasm in the region of 

 the Qg^ from which they arise. Thus, in all telolecithal ova, where 

 the cleutoplasm is mainly stored in the lower or vegetative hemi- 

 sphere, as in many worms, mollusks, and vertebrates, the cells of the 

 upper or protoplasmic hemisphere are smaller than those of the lower, 

 and may be distinguished as inicromcrcs from the larger macronicrcs 

 of the lower hemisphere. The size- ratio between micromeres and 

 macromeres is on the whole directly proportional to the ratio between 

 protoplasm and deutoplasm. Partial or discoidal cleavage occurs 

 when the mass of deutoplasm is so great as entirely to prevent cleav- 

 age in the lower hemisphere. 



/^. 



B 



Fig. 124. — • Partial or meroblastic cleavage in tlie squid Loligo. [Watase.] 



Balfour's law undoubtedly explains a large number of cases, but 

 by no means all ; for innumerable cases are known in which no cor- 

 relation can be made out between the distribution 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 inapplicable to the in- 

 equalities of the first two divisions in annelids and gasteropods. It 

 is conspicuously inadequate in the history of individual 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 (.V, Fig. \22, E), undergoes an inva- 

 riable order of division, three unequal divisions being followed by 

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

 equal. This cell contains no deutoplasm and undergoes no percepti- 



