366 CELL-DIVISION AND DEVELOPMENT 



1. Variation in the rhythm of division. 



2. Displacement of the cells (including variations in the direction 

 of cleavage}. 



3. Unequal division of the cells. 



Nothing is more common than a departure from the regular 

 rhythm of division. The variations are sometimes quite irregular, 

 sometimes follow a definite rule, as, for instance, in the annelid Nereis 

 (Fig. 171), where the typical succession in the number of cells is with 

 great constancy 2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, after which 

 the order is more or less variable. The factors that determine such 

 variations in the rhythm of division are very little understood. Bal- 

 four, one of the first to consider the subject, sought an explanation in 

 the varying distribution of metaplasmic substances, maintaining ('75, 

 '80) that the rapidity of division in any part of the ovum is in general 

 inversely proportional to the amount of deutoplasm that it contains. 

 The entire inadequacy of this view has been demonstrated by a long 

 series of precise studies on cell-lineage, which show that while the 

 large deutoplasm-bearing cells often do divide more slowly than 

 the smaller protoplasmic ones the reverse is often the case, while 

 remarkable differences in the rhythm of division are often observed 

 in cells which do not perceptibly differ in metaplasmic content. 1 All 

 the evidence indicates that the rhythm of division is at bottom deter- 

 mined by factors of a very complex character which cannot be 

 disentangled from those which control growth in general. Lillie 

 ('95, '99) points out the very interesting fact, determined through an 

 analysis of the cell-lineage of mollusks and annelids, that the rate 

 of cleavage shows a direct relation to the period at which the prod- 

 ucts become functional. Thus in Unio the more rapid cleavage of a 

 certain large cell ("d. 2"), formed at the fourth cleavage, is obviously 

 correlated with the early formation of the shell-gland to which it gives 

 rise, while the relatively slow rate of division in the first ectomere- 

 quartet is correlated with reduction of the prae-trochal region. The 

 prospective character shown here will be found to apply also to other 

 characters of cleavage, as described beyond. 



When we turn to the factors that determine the direction of cleav- 

 age or the displacement of cells subsequent to division, we find, as 

 in the case of the division-rhythm, obvious mechanical factors com- 

 bined with others far more complex. The arrangement of tissue-cells 

 usually tends toward that of least resistance or greatest economy of 

 space ; and in this regard they have been shown to conform, broadly 

 speaking, with the behaviour of elastic spheres, such as soap-bubbles 

 when massed together and free to move. Such bodies, as Plateau 



1 Cf. Wilson, '92, Kofoid, '94, Lillie, '95, Zur Strassen, '95, Ziegler, '95, and especially- 

 Jennings, '97. 



