EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 73 



ceeding cleavages, but in cases of some eggs which have been 

 exposed to the X-rays I have noticed that the second furrow is 

 the one to cut in more deeply. 



When the nuclei of the blastomeres have moved to the center 

 they begin the second division. By the time the cytoplasmic 

 division can be seen the nuclear division is well under way. It is 

 the rule to which exceptions are only occasionally seen that the 

 two cells should divide at the same time. However, it is true 

 that one cell will sometimes divide before the other has pro- 

 gressed far, and a three-cell condition results as shown in Plate I., 

 Fig. 8, of Conklin's ('97) paper. 



The blastomeres again elongate in the direction of the long 

 axis of the spindle. The furrow begins to cut in from the sides 

 and from the animal poles. The cells again round off after the 

 division, and we again find the cleavage cavity present. It 

 differs from the first cleavage cavity, however; in addition to the 

 lenticular spaces which appear between each pair of dividing 

 cells, a large rectangular cavity is to be seen at the center. 



The third cleavage consists in the giving off of the first quartette 

 of micromeres (ectomeres) at the animal pole. A spiral dextro- 

 tropic shifting of the quartette occurs after which the cells flatten; 

 and there remains once more an almost spherical mass of cells. A 

 central cleavage cavity again makes its appearance and again 

 disappears after the next cleavage. 



The four macromeres divide now to produce a second quartette 

 of micromeres ; thus arises the twelve-cell stage. As before, after 

 the division the cells flatten to form a spherical mass. In the 

 living condition it is most easy to determine which stage is being 

 observed when one can see the egg in an optical section running 

 from pole to pole. The division of the first quartette of ectomeres 

 now produces the sixteen-cell stage which in turn give rise to the 

 twenty-four-cell stage by division of the second quartette of 

 ectomeres and at the same time a division of the macromeres 

 by which the third quartette of ectomeres is given off. According 

 to Holmes "the twenty-four-cell stage, which is reached by these 

 divisions, marks a resting stage of considerable length in the 

 development of the egg. A cleavage cavity is formed at this time 

 which may acquire quite a large size" (page 380). This is the 



