32 EDWAED MCCRADY, JR. 



smaller blastomere averages about 0.082 mm. X 0.061 mm. in 

 its longest and shortest diameters, respectively; whereas the 

 larger averages 0.099 mm. X 0.070 mm. These measurements 

 indicate a discrepancy of about 26% by volume, the larger 

 blastomere containing 63%, the smaller 37% of the total 

 volume of the egg. This information is useful in solving the 

 question of cell lineage at the next stage. 



The nuclear difference between the two poles of the egg re- 

 appears when the spindles form for the second division. The 

 two eggs of The Wistar Institute collection which show these 

 spindles (no. 16258 and no. 16275) are both anaphases, and in 



Fig. 10 Section of 2-celled ovum 16258. Photograph from Hartman ('19). 

 Showing spindles for second cleavage, and yolk granules in the perivitelline space. 



each case the chromosomes in the larger blastomere are 

 further advanced than those in the smaller. Furthermore, 

 the spindles are not parallel in the two blastomeres, but are 

 approximately at right angles to each other. Both are also 

 at right angles to the position of the original spindle in the 

 first cleavage (fig. 10). 



From this point on for the sake of brevity I shall call the 

 larger blastomere of the 2-celled stage A and the smaller B. 

 Before the cleavage furrow has appeared in B, A divides 

 meridionally, so that a 3-celled stage (stage 3) is formed, 

 composed of A 1? A 2 and B. B then divides producing a 

 4-celled stage (stage 4) which, in accord with the difference 



