62 FRANK R. LILL1E. 



with reference to the primary axis, the first cleavage is invariably 

 truly meridional, and the proportions of the cells are usually 

 normal or approximately so. In some cases the small cell is 

 packed full of yolk granules, in. other cases practically devoid of 

 them ; nevertheless, the first cleavage is substantially unaffected 

 as to direction or proportions by the direction of stratification 

 (see Lillie, 1906, p. 199). 



2. Is the bilateral organization of the egg a property of the 

 primary ovocyte like polarity, or does it arise in the course of 

 development after rupture of the germinal vesicle ? The evi- 

 dence seems to me in favor of the latter proposition for the fol- 

 lowing reasons : 



(#) If the ground substance is bilaterally organized before 

 maturation there should be morphological evidence of it, as there 

 is of the existing polarity. But there is none whatever ; the 

 structure of the egg is radially symmetrical in the plane at right 

 angles to the polar axis until the close of maturation. After 

 the union of the germ nuclei, the bilateral symmetry comes 

 to expression slowly by granule movements that cause a pre- 

 ponderant distribution of the yolk granules on one side of 

 the egg, which becomes the posterior side, and by correspond- 

 ing displacement of the yolk-free protoplasm and spindle towards 

 the opposite side of the egg, which becomes the anterior side. 

 This is the case in both Ch&topterus and Nereis, as well as in 

 Unio. If the conditions causing such displacement existed 

 prior to maturation, one would expect that they would become 

 effective at an earlier period. 



(b] In the summer of 1908 I found that it was possible to 

 break up the eggs of Ch&topterus into fragments of varying sizes 

 by high centrifugal force. The protoplasm of this egg is much 

 more fluid than that of any other egg I have tested ; in conse- 

 quence, the egg elongates even with relatively low centrifugal 

 speed ; and with very high speed the eggs break apart along the 

 lines of stratification. Thus one obtains numerous perfectly 

 hyaline pieces composed of the substance of the clear band, and 

 others densely packed with yolk granules. The nucleus is almost 

 invariably contained in the former kind, but it may occur in the 

 latter, owing to the fact that at the time of centrifuging the matu- 



