256 



BIOLOGICAL LECTURES. 



rical cleavage is necessary for the symmetrical distribution of 

 the precociously segregated material, so that it is to be expected 

 that the axes of symmetry in the cleaving egg should correspond 

 with the axes of symmetry in tJie adult. It is probable, however, 

 that the establishment of these axes is significant as an incident 

 connected with the mechanics of this form of cleavage, rather tJian 

 as indicating differentiation. 



The time of division in the different blastomeres is also 

 a factor in the result attained. As mentioned above, the time 

 at which the spiral form of cleavage ends is closely connected 

 with the process of gastrulation, etc. All through the bilateral 

 period of cleavage the divisions seem to occur at the right time 

 to serve as a factor in morphogenesis. In the case of symmet- 

 rical divisions on the two sides of the egg this fact is especially 

 noticeable, and the best illustration is found in the later divisions 

 of the somatic plate (Figs. 11-17). The symmetrical divisions 

 occur at the same time, or nearly, although they may be almost 

 on opposite sides of the egg. Sometimes one side is slightly 

 ahead of the other, but in any case the variation is not great, 

 and the arrangement of the material is the same in every case. 

 In general, it may be stated that the larger cells divide more 

 rapidly than the smaller, but this rule does not hold good in 



all cases, and does not apply of 

 course to yolk-laden cells. In- 

 deed, as Conklin ('97) and Lillie 

 ('95) have suggested, the rapidity 

 of division is apparently regulated 

 in some cases by the time at which 

 the portion of material concerned 

 is to become functional, but 

 neither does this rule apply to all 

 cases. For instance, the primary 

 trochoblasts in Arenicola are rel- 



FIG. 23. Clymenella. Small size of cross cells <- ~U. Crrm l1p r f-V,o n i n A>ni<hJii 

 and large size of primary trochoblasts (after atlVely L Amptll- 



Mead )- trite and Clymenella (cf. Figs. 7 



and 23), and become ciliated at a much later stage. Yet in 

 the two forms the trochoblasts pass through all of their divi- 

 sions with equal rapidity, i.e., relatively to the other blasto- 



