2o6 BIOLOGICAL LECTURES. 



That the environment of a cell due to its position in the egg 

 does not account for its rate of cleavage in the egg of Amphi- 

 trite would appear from the following : (i) adjacent cells may- 

 have entirely different rates of cleavage, the one dividing not 

 at all, the other dividing rapidly; (2) cells which occupy exactly 

 corresponding positions in different quadrants of the Q.g^ ex- 

 hibit great diversity in the rate of cleavage ; (3) the power of 

 dividing is suddenly lost in the cells which form the prototroch 

 — there is not a gradual waning of karyokinetic activity in the 

 successive generations preceding these particular blastomeres ; 



(4) the rate of division in the various 

 cells is the same in whatever position 

 the Q^^ may lie. 



Moreover, in related eggs of the 

 same cleavage type, certain blasto- 

 meres have a very different environ- 

 ment by virtue of the difference in 

 the absolute and in the relative size 

 of their neighbors, but the rate of 

 cleavage does not vary accordingly. 

 Fig. 2. — Egg of Scoiecoiepis from Thus, the cclls which form the proto- 



above. The four cells at the animal . i • • r^^ 



pole undivided; the yoik-iadeu cells troch m Amphitritc, Clymcnella, and 

 atjegetative pole undergoing divi- Arcuicola, respcctivcly, arc different 



in their absolute and in their relative 

 size ; yet the cessation of division occurs at exactly the same 

 period of cell development in all three annelids, 



Balfour's generalization that the divisions occur more fre- 

 quently or less frequently according as the cells contain little 

 or much yolk in proportion to the protoplasm has been shown 

 by many observers to be of only limited application. One of 

 the annelids well illustrates the inadequacy of this "law," In 

 the 8-cell stage of Scoiecoiepis the four lower blastomeres con- 

 tain all the yolk of the o.^^ and are many times larger than the 

 four upper blastomeres, which are free from yolk; and yet the 

 four yolk-laden cells divide sooner than those which contain 

 only pure protoplasm (Fig. 2). 



The same differences in the rate of division that exist in 

 the early cleavage of the ovum obtain in the later stages of the 



