geOaMEtrical relations of cleavage-forms 375 



eccentricity, the greater the inequaUty, an extreme form being beauti- 

 fully shown in the formation of the polar bodies. Here the original 

 amphiaster is perfectly symmetrical, with the asters of equal size 

 (Fig. 97, A). As the spindle rotates into its radial position and 

 approaches the periphery, the development of the outer aster be- 

 comes, as it were, suppressed, while the central aster becomes enor- 

 mously large. The size of the aster, in other zvords, depends upon the 

 extent of the cytoplasmic area that falls zvithin tJie sphere of influence 

 of the ccntrosome ; and this area depends upon the position of the 

 centrosome. If, therefore, the polar amphiaster could be artificially 

 prevented from moving to its peripheral position, the ^^^ would 

 probably divide equally. ^ 



This leads us to a further consideration of the attempts that have 

 been made to explain the movements of the mitotic figure through 

 mechanical or other causes.^ Highly interesting experiments have 

 been made by Pfluger ('84), Roux ('85), Driesch ('92), and a number 

 of later investigators which show that the direction of cleavage may 

 be determined, or at least modified, by such a purely mechanical 

 cause as pressure, through which the form of the dividing mass is 

 changed. 



Thus, Driesch has shown that when the eggs of sea-urchins are 

 flattened by pressure, the amphiasters all assume the position of least 

 resistance, i.e. parallel to the flattened sides, so that the cleavages 

 are all vertical, and the Q.g^ segments as a flat plate of eight, sixteen, 

 or thirty-two cells (Fig. 186). This is totally different from the nor- 

 mal form of cleavage ; yet such eggs, when released from pressure, 

 are capable of development and give rise to normal embryos. This 

 interesting experiment makes it highly probable that the disc-like 

 cleavage of meroblastic eggs, like that of the squid or bird, is in some 

 degree a mechanical result of the accumulation of yolk by which the 

 formative protoplasmic region of the ovum is reduced to a thin layer 

 at the upper pole ; and it indicates, further, that the unequal cleavage 

 of less modified telolecithal eggs, like those of the frog or snail, are 

 in like manner due to the displacement of the mitotic figures toward 

 the upper pole. 



The results of Pfliiger's and Driesch's pressure experiments obvi- 

 ously harmonize with Hertwig's second rule, for the position of least 

 resistance for the spindle is obviously in the long axis of the proto- 

 plasmic mass which is here artificially modified ; and it harmonizes 

 further with Drliner's hypothesis of the active elongation of the 

 spindle in mitosis (p. 105). There are, however, a large number of 

 facts which show that neither the form of the protoplasmic mass nor 



1 Cf. Francotte on the polar bodies of Turbellaria, p. 235. 



2 For a good review and critique, see Jennings, '97. 



