CLEAVAGE IN TOXOPNEUSTES VARIEGATUS. 14! 



cells as regards size, these being respectively, the small macro- 

 meres, a group of eight medium-sized cells at the animal pole; 

 the micrometers, four small cells at the vegetal pole; and t*he 

 large macromeres, four large cells lying between the two groups 

 just mentioned. These are shown as seen from the side in 

 Fig. 8 and as seen from the vegetal pole in Fig. 9. It is one hour 

 and fifty minutes after the addition of the sperm when this stage 

 is complete. 



From this time on, all of the cells do not divide simultaneously. 

 Two hours and ten minutes after insemination the four large 

 macromeres are divided vertically into eight, and the eight 

 small macromeres are divided vertically into sixteen. About 

 ten minutes later the micromeres are cut horizontally into eight, 

 but this is an unequal division, the four smaller micromeres lying 

 nearer the vegetal pole (Fig. 10). The completed thirty-two- 

 celled embryo is shown from the side in Fig. n, and the crowding 

 in of the micromeres can be noticed at this stage. 



The next furrow cuts the lower set of eight large macromeres 

 horizontally, and twenty minutes later, or two hours and fifty-five 

 minutes after insemination, vertical cleavages divide the upper 

 set of large macromeres into sixteen, and the sixteen small 

 macromeres into thirty-two. After three hours and five minutes, 

 the upper micromeres are divided vertically into eight, the four 

 micromeres referred to above as lying nearer the vegetal pole, 

 now being crowded back and partially concealed by this ring of 

 eight cells. Five minutes later the lowest set of macromeres is 

 divided by vertical cleavages into sixteen (Fig. 12). Twelve of 

 the seventy-six cells of this stage are micromeres, the later 

 development of which will be taken up presently. 



The following table (page 142) shows in a diagrammatic way, 

 the sequence of cleavage in the Toxopneustes egg. The time 

 indicated is the elapsed time after insemination. 



Theel's (1892) account for Echinocyamus pusillus differs from 

 this in that he describes the micromeres as being cut off at the 

 animal pole. According to his description the sixteen-cell stage is 

 completed in about two hours and twenty minutes and is accom- 

 plished as follows: "First the four upper segments are divided 

 by a horizontal plane into four small cells or micromeres and 



