Organisms from Eggs 



137 



Fig. 15 



other (Fig. 15), as the writer had a chance to observe 

 in the egg of Strongylocentrotus purpuratus^ in the fol- 

 lowing experiment. The eggs of the sea urchin Strongy- 

 locentrotus purpuratus are put soon after fertilization 

 into solutions which differ from sea water in two 

 points ; namely that they are neutral or very faintly acid 

 (through the C 2 

 absorbed from the 

 air) instead of being 

 faintly alkaline, and 

 second, that_ one of 

 the following three 

 constituents of the 

 sea water is lacking; 

 namely: K, Na, or Ca. When the eggs are allowed to 

 segment in such a solution the first two cleavage cells 

 are as a rule in a large percentage of cases — often as 

 many as ninety per cent. — separated from each other, 

 and when the eggs are put into normal sea water (about 

 twenty minutes after the cell division) each cell develops 

 into a normal embryo. In a number of cases the em- 

 bryos remained inside the egg membrane and did not 

 move until after the invagination of the intestine was 

 far advanced; in such cases it was found quite often 

 that the invagination began at the plane of cleavage at 

 symmetrical points of the two embryos, and the growth 

 of the intestine was symmetrical in both embryos. 

 ' Loeb, J., Arch.f. Entwcklngsmech., 1909, xxvii., 119. 



