34 EDWARD MCCRADY, JR. 



volume as blastomere B of the 2-celled stage is actually blasto- 

 mere B still undivided, and that the two smaller cells are 

 the descendants of blastomere A, which lost 51,079 u 3 of its 

 volume by discharging that amount of yolk into the 'peri- 

 vitelline' space during division. 



The 4-celled stage (stage 4) is produced by the division of 

 blastomere B. It divides meridionally, but at right angles to 

 the plane of division of A; and, in doing so, it in turn dis- 

 charges yolk while blastomeres A x and A 2 remain quiescent. 

 In this manner there is a restoration of the original volume 

 relations of the A and B poles of the egg. That is to say, in 



Fig. 11 Photograph of wax model of 16275, showing yolk granules protruding 

 through the surface of the blastomeres. The needles run through the poles of 

 the mitotic spindles and show their angle of inclination to each other. 



the 4-celled stage the daughter cells of A taken together again 

 represent about 60% of the total volume of the egg; and those 

 of B about 40%. 



It should be mentioned that Hartman ('19) believed that 

 the spindles in A and B are parallel when they first form, 

 and subsequently rotate during division. His opinion was 

 based upon an egg (no. 306.2 of the Hartman series, now 

 no. 16275 of The Wistar Institute collection) in which he con- 

 sidered the spindles to be parallel. I have made both a glass 

 plate reconstruction and a wax plate reconstruction (fig. 11) 

 of this egg, and both show this not to be the case. The 



