THE PHYSIOLOGY OF CELL DIVISION 31 



formed by a surface-condensation of certain protoplasmic con- 

 stituents, particularly those which lower surface tension. The 

 starfish egg is deficient in cholesterin according to Mathews, ^^ 

 and this circumstance may be important, since the presence of 

 cholesterin seems in general to render plasma membranes rela- 

 tively resistant to chemical alteration. ^^ The plasma membrane 

 of the mature starfish egg seems unusually sensitive to changes in 

 the surrounding medium, and the responsiveness of these eggs 

 to the above form of after-treatment is probably an expression 

 of this peculiarity. The experiments now about to be described 

 relate entirely to starfish eggs. 



II. EXPERIMENTAL 



In all the experiments described below the unfertilized eggs of 

 starfish (Asterias forbesii and vulgaris) were used.^^ The eggs 

 were first subjected to a membrane-forming treatment; then, 

 after an interval of ten minutes in normal sea-water, they were 

 exposed for thirty minutes to sea-water containing anesthetics in 

 the concentrations given below. For comparison, part of the 

 eggs remained in sea-water without after-treatment, and part 

 were exposed for thirty minutes to hypertonic sea-water and to 

 sea-water containing potassium cyanide in m/1000 concentration. 

 The eggs were then returned to normal sea-water and on the fol- 

 lowing day the proportion developing to a free-swimming larval 

 stage was approximately determined. 



22 A. p. Mathews, unpublished observations made at Woods Hole. 



23 Cf. Hober, Physikalische Chemie der Zelle und der Gewebe. 3rd Ed., 1911, 

 p. 232. 



2^ In 1912 good starfish eggs were abundant at Woods Hole throughout the 

 summer. A difference was observable between the eggs obtained toward the mid- 

 dle and end of June, and those obtained later (from July on). The latter showed 

 the more regular behavior and were in general more responsive, while eggs ob- 

 tained towards the end of June were frequently resistant to fertilization and 

 showed irregularities of behavior similar to those described in my recent paper 

 in tte Biological Bulletin (1912, vol. 22, p. 328). Two maxima of egg-production 

 are thus indicated, which probably correspond to the two specific types, A. for- 

 besii-and A. vulgaris, recognized as occurring in this region. In the expriments 

 described below the majority of the eggs showed normal behavior in regard to 

 post-maturational cytolysis and response to fertilization. 



