CELL DIVISION IN EGGS OF CREPIDULA. 533 



effect on the eggs of Crepidula. However, when equal parts of fresh water and 

 sea water were used, many modifications were produced. Almost all of the ex- 

 periments enumerated above were made with a dilution of equal parts of fresh 

 water from the tap and of the normal sea water supplied to the aquaria at 

 Wood's Hole. In three of the experiments, viz., Nos. 859, 875, 876, 100 c.c. of 

 fresh water were added to 50 c.c. of normal sea water. The eggs were left in 

 the diluted sea water for lengths of time varying from 3^2 hour to 7}/£ hours and 

 were either fixed at once or put back into normal sea water for periods varying 

 from 3^ hour to 52 hours. 



Driesch (1893) found that sea urchin eggs did not segment in 30 parts normal 

 sea water and 20 parts distilled water, though the nuclei continued to divide. 

 Loeb (1895 1 ) was able by means of diluted sea water to cause the egg membrane 

 of the Arbacia egg to burst and a portion of the egg to flow out, and from such 

 eggs double embryos developed. Schucking (1903) caused artificial parthen- 

 ogenesis in Asterias by the use of distilled water. 



The first effect of the diluted sea water on the eggs of Crepidula is to cause 

 a slight swelling of the cells and to render all cell membranes very indistinct. 

 Indeed the outlines of eggs become hazy and the macromeres sometimes separate 

 from one another more or less completely. In all cases where eggs in dilute sea 

 water are crowded together, they lose their individual outlines and apparently 

 fuse together. If the diluted sea water is allowed to act for a long time or if the 

 dilution is great (2:1), the cell membranes may burst. Even in extreme cases, 

 however, the swelling of the eggs in diluted sea water is not great; when fixed 

 and mounted the diameter of the normal egg after fertilization and before cleavage 

 is about 136-140ju. The swollen egg rarely measures more than 144/*. After 

 the eggs have been returned to normal sea water they again shrink to normal 

 size, and cell membranes become normal in appearance. 



The eggs shown in figs. 132-135, 137-140, 144, were placed for ^ hr. in sea 

 water diluted with two volumes of fresh water, and were then left in normal sea 

 water for 7 hrs. All other eggs figured on Plates LIII and LIV were treated with 

 sea water to which was added equal parts of fresh water. 



In all of these eggs, development has been delayed more or less. Eggs 

 which were put into diluted sea water during maturation stages are frequently 

 polyspermic (figs. 132-134) ; the supernumerary sperm nuclei are evidently under- 

 going degeneration, since they are homogeneous and stain faintly. 



In considering the effects of diluted sea water on cell division, the effects on 

 the division of the cell body will be considered first, and then the effects on mitosis. 

 In figs. 132-135 and 145-148 division of the yolk has been completely suppressed; 

 a very large second polar body is shown in fig. 134, and various stages in the 

 cleavage of the protoplasmic portion of the egg are shown in figs. 145-148. The 

 arrangement of the micromeres in the figures last named is more or less irregular, 

 and it is not possible to tell whether three micromeres were separated from the 

 single large macromere in sets of one, or whether the micromeres were separated 

 from the macromere in sets of two or more. Since polyasters were present in 



