290 



CELL-DIVISION AND DEVELOPMENT 



ber of cells in the blastula just before invagination is approximately 

 proportional to the size of the blastula, though the smaller frag- 

 ments show a tendency to produce a somewhat larger number, and 

 their cells are, therefore, somewhat smaller than in the larger forms. 

 The same is true in AmpJiioxus. Morgan, therefore, draws the very 

 interesting conclusion that " the ultimate size of the cells produced 

 by repeated division determines when the division shall come to 

 an end for a certain stage of the ontogeny." ^ This conclusion 

 is, however, subject to exception ; for Morgan finds that the dwarf 

 larva; show a tendency to use the same number of cells in the 

 formation of certain organs as the full-sized individuals. Thus the 

 dwarf blastulas tend to invaginate the same number of cells to 

 form the archenteron as in the normal forms; and in Arnphioxus 

 the notochord of the dwarfs consists in cross-section of three cells, 

 as in normal individuals, irrespective of the total number of cells. It 

 is clear, therefore, that there is another factor, besides the size of the 

 cells, to be taken into account, and the whole subject awaits further 

 investigation. 



The gradual diminution of the energy of division during develop- 

 ment by no means proceeds at a uniform pace in all of the cells, and, 

 during the cleavage, the individual blastomeres are often found to 

 exhibtt entirely different rhythms of division, periods of active division 

 beino- succeeded by long pauses, and sometimes by an entire cessa- 

 tion of division even at a very early period. In the echinoderms, 

 for example, it is well established that division suddenly pauses, or 

 changes its rhythm, just before the gastrulation (in Synapta at the 

 512-cell stage, according to Selenka), and the same is said to be 

 the case in Arnphioxus (Hatschek, Lwoff). In Nereis, one of the 

 blastomeres on each side of the body in the forty-two-cell stage 

 suddenly ceases to divide, migrates into the interior of the body, 

 and is converted into a unicellular glandular organ. ^ In the same 

 animal, the four lower cells (macromeres) of the eight-cell stage 

 divide 'in nearly regular succession up to the thirty-eight-cell stage, 

 when a long pause takes place, and when the divisions are re- 

 sumed they are of a character totally different from those of the 

 earlier period. The cells of the ciliated belt or prototroch in this and 

 . other annelids likewise cease to divide at a certain period, their num- 

 ber remaining fixed thereafter.^ Again, the number of cells produced 

 for the foundation of particular structures is often definitely fixed, 

 even when their number is afterwards increased by division. In 



1 '95, p. 119. 



2 This organ, doubtfully identified by me as the head-kidney, is probably a mucus-gland 



(Mead). 



3 Cf. Fig. 122. 



