124 FRANCIS MARSH BALDWIN. 



fertilization. He found also in acid and salt solutions (pure 

 isotonic KCL and NaCL) a similar but less clearly denned rhythm 

 of susceptibility. Eggs subjected to heat, electrical stimulation 

 and hypertonic sea-water behave in a similar manner. Thus, 

 Lyon, 1 observed that the eggs were most resistant to heat at a 

 time previous to the first cleavage, and were most readily injured 

 at the time of division. A. R. Moore 2 finds that the resistance 

 to hypertonic sea-water is least "immediately before and during 

 each cytoplasmic division, and that the maximal resistance is 

 shown 35 to 45 minutes after fertilization and just after each 

 division." More recently Lillie 3 (1916), has made an extensive 

 study of the rhythmical changes in the resistance of the dividing 

 sea-urchin egg to hypotonic sea-water, and has discussed the 

 physiological significance of this rhythm. His experiments 

 show clearly that at or about the time of formation of the cleavage 

 furrow, a marked decline takes place in the resistance of the egg 

 to hypotony, and cytolysis is then rapid and complete. After 

 the cleavage furrow is fully formed the original resistance returns. 

 A similar reversible decline of resistance takes place at the second 

 and third cleavage, and is probably general for mitotic cell- 

 division. The minimum of resistance is found during the for- 

 mation of the furrow. Both the decline and the return of resis- 

 tance are rapid, the greater part of each phase occupying four 

 to five minutes. Some increase of susceptibility is apparent 

 ten or twelve minutes before the first appearance of the furrow. 

 Similar observations have been made by Herlant 4 in the egg of 

 Paracentrotus lividus. 



From such experiments it appears that the resistance of the 

 eggs to a variety of injurious agencies is least at the time when 

 they are undergoing rapid change of form. To account for 

 these rhythmical changes in the physiological state of the egg, 

 Lillie 4 (1909) puts forward the hypothesis that they are essentially 

 the result of variations in the physical condition, especially the 

 permeability, of the surface-film of plasma-membrane, the latter 



1 E. P. Lyon, Atner. Jour. Physiol., 1904, Vol. 11, p. 52. 



2 A. R. Moore, BIOL. BULL., 1915, Vol. 28, p. 257. 



3 R. S. Lillie, Jour. Exper. Zoo/., 1916, Vol. 21, No. 3, p. 401. 



4 M. Herlant, Comptes rendus d. I. Societe d. Biologic, 1918, Vol. 81, p. 151. 

 6 R. S. Lillie, BIOL. BULL., 1909, Vol. 17, p. 207. 



