LIMITS OF DIVISIBILITY OF LIVING MATTER 327 



vate and the egg. Four hours later this egg was in the 

 condition shown in Fig. 76. The cleavage cells lying within 

 the egg in Fig. 73 have developed into the blastula c. The 

 micromeres and the large cells inclosed between them in Fig. 

 75 have developed into a misshapen mass of cells d in Fig. 

 76 ; each two of the four large cells of the extraovate have led 

 to the development of the separate blastulsB a and &, so that 

 we obtained from this one egg three blastulse and a mass of 



FIG. 77 



FIG. 78 



shapeless cells. 1 Fig. 77 shows the same egg twenty-four 

 hours later. The largest of the blastulse c, which has remained 

 within the egg-membrane, has developed into a gastrula, 

 while the two smaller blastulse a and 6, which have remained 

 outside the egg, have developed no further. A short time 

 after this drawing was made all four pieces began to swim 

 about in the drop. The formation of the blastula therefore 

 occurred at the same rate in the smaller masses as in the 

 larger one. I may add that it also occurred at the same rate 

 as in the eggs whose membrane had not burst. One notices, 

 of course, that eggs which are placed into dilute sea-water, 

 and consequently go into "water rigor," do not all recover 

 and begin to segment at the same time after they are returned 

 to normal sea-water. Under these conditions an extraovate 



i It often happened that the cells of the extraovate formed not one, but two or 

 more, blastulse. The sliding motions of the cells are not restricted in the extraovate, 

 and can therefore lead to various groupings of the cell-masses. Inside of the egg- 

 membrane this origin of twins also occurs, but more rarely. The membrane restricts 

 the sliding motions of the cells. I shall discuss this question in greater detail later 

 in a paper on the formation of double embryos. 



