1 6 Edmund B. Wilson. 



accompanied by an extensive shifting of the cytoplasmic mater- 

 ials in all of the cells. In the three basals, A, B and C, the white 

 material towards the animal pole moves over towards the upper 

 right angle of the cell and Increases in amount, extending so far 

 down the egg that in some individuals it may be seen, when the 

 egg is viewed from the vegetative pole, as a narrow white cres- 

 centic area (Fig. i6). A similar process takes place in D, but 

 in addition to this a great change takes place in the white material 

 of the lower polar area, which leaves its position at the lower pole, 

 moves over towards the same side as the upper white area, and 

 finally fuses with it, while the pigmented part becomes lighter in 

 color, often irregular or mottled in appearance, and extends into 

 the area formerly occupied by the lower white substance. In 

 the ensuing cleavage, D is usually the first to divide, giving 

 rise by a leiotropic cleavage to the large first somatoblast, 2d or 

 X (Figs. 17, 18). This cell consists almost entirely of white 

 material which is certainly derived in large part from the orig- 

 inal lower white area, but undoubtedly also in part from the upper 

 white area, which, as stated above, fuses with the lower area in 

 the period preceding this cleavage. In some cases X receives 

 also a small amount of the pigment (Fig. 18), in others it seems 

 to be composed entirely of white material. The other members 

 of the second quartet, 2a, 2b, and 2c, are much smaller than X, 

 and each is formed mainly from the white material of the upper 

 polar area, but as a rule, perhaps always, each receives also a 

 variable amount of pigment. During the foregoing changes the 

 upper quartet divide leiotropically in the usual fashion, to form 

 the four primary trochoblasts, which are slightly smaller than 

 the upper cells. Owing to the foregoing changes the pigment, 

 which in the unsegmented egg extended far up towards the animal 

 pole, has been moved downwards so as to lie below the 

 equator of the egg, most of it being contained in A, B and 

 C, some in D, a little in 2a, 2b and 2C, and sometimes also a little 

 in 2d. The pigment becomes still more restricted during the 

 fifth cleavage, since the micromeres of the third quartet are again 

 mainly composed of white substance. 



