66 ILLINOIS BIOLOGICAL MONOGRAPHS [260 



the naked eye, the egg was a reddish-brown speck just barely visible, and ap- 

 pearing quite dark. Under some magnification and with reflected light, the 

 yellow was much stronger altho the influence of the white internal portion of 

 the egg was more noticeable. The shell though rather thick and heavy is 

 quite transparent and clearly shows the yolk granules in the younger stages 

 and the details of embryonic structure in the later. 



Figure 10 shows the egg as it appears during the first few hours before seg- 

 mentation has begun. The yolk granules give a cellular appearance and near 

 the center can be seen the nucleus, rather large and with a heavy wall. The 

 granules vary in size, some being not more than one-quarter as large as others. 

 They are shghtly milky, somewhat transparent, almost a pure white, irregular 

 in shape, angular, due partly to pressure, and closely packed. When the egg 

 is broken, the granules escape into the water in a mass, the individuals adhering 

 closely, and holding their shape even when separated. Very Uttle change could 

 be noticed during the first six hours, but soon after that the even appearance 

 was lost, as at different points there seemed to be a breaking or drawing apart 

 of the granules, showing lighter streaks. During the first twelve hours, the 

 first and second cleavage stages were finished. The divisions were not dis- 

 tinct, showing very clearly only at the edge where a notch marked the rounding 

 of the cells. 



By the end of the first thirty-six hour period, the cell divisions were much 

 more distinct, the cells had attained a spherical shape and had arranged them- 

 selves definitely. The macromeres could be distinguished drawn to one side 

 of the egg while the four micromeres appeared toward the center and opposite 

 side. At different times the nuclei of all the cells were visible as small, round 

 shadows. The spaces left by the formation and shaping of the cells became 

 more transparent so that the outlines were distinct and the whole aspect of 

 the egg was changed, i.e., the cells were adhering to each other only slightly, 

 but still in one integral mass, and were floating in the surrounding fluid. The 

 macromeres were very large, about one-sixth the size of the egg and were 

 arranged as a hollow plate, while the micromeres, somewhat smaller, lay curved 

 inside. By turning the egg slightly different views of the various cells could 

 be obtained, and their relations made out. 



During the next few hours, the division stages continued. In many cells, 

 the new waUs could still be distinguished but the mass soon became so solid 

 and opaque that it was impossible to follow the cell lineage with any degree 

 of accuracy. The two poles of the egg were marked by the difference in cells 

 which at the micromere or animal pole increased in numbers, and soon filling it 

 began to push down as a cap over the large macromeres. At the beginning 

 of the third day the cap of small, compact cells had extended more than half 

 way over the egg, giving a typical gastrula stage. This was very conspicuous 

 and characteristic, as the cells at the animal pole were dark and heavy, while 

 the free portion at the opposite end was clear and transparent. During the 

 third and fourth days the mass of cells in some places grew to extend all the 

 way around to the ventral side. 



