PHYSIOLOGY OF CELL-DIVISION 387 



cannot be assigned at present with any certainty; there is, how- 

 ever, some evidence, partly cited above (p. 375), that resistance 

 to hypotony does not necessarily depend upon the same condi- 

 tions as resistance to cytolytic substances; thus in the case of 

 blood-corpuscles, there appears to be an inverse relation between 

 the two kinds of resistance — i.e., those corpuscles (e.g., of the 

 rabbit) which are most readily cytolyzed by saponin, resist 

 best the exposure to hypotonic media. As already stated, such 

 corpuscles appear to be relatively deficient in cholesterol. It 

 may be that those membrane-constituents which prevent the 

 entrance of water-soluble substances and water (possibly chole- 

 sterol?) are diminished immediately after fertilization, while 

 those which impart coherence to the membrane (e.g., lecithin?) 

 remain unchanged; on this view the latter membrane-components 

 would diminish at the time of cleavage; the question, however, 

 cannot be settled without further investigation. The fact 

 remains that the membrane preserves unaltered its resistance 

 to osmotic disruption throughout the entire period succeeding 

 fertilization, until shortly before the egg begins to cleave. 



Properties of the fertilization-membrane 



Certain incidental observations on the formation of extra- 

 ovates are cited in table 3, since they appear to throw some 

 light on the vexed question of the nature of the fertilization- 

 membrane. When fertilized eggs are placed in dilute sea-water, 

 the fertilization-membrane is often ruptured by the pressure 

 of the distended egg, and the protoplasm partly flows out, the 

 egg assuming the shape of an hour-glass.^" It was always found 

 that these extra-ovates were much more readily formed in eggs 

 brought into dilute sea-water immediately after fertilization 

 (within two minutes) than later. Table 6 gives the results 

 of seven experiments in which the proportion of extra-ovates 

 was estimated in eggs placed in dilute sea-water at 2, 4 and 6 

 minutes after fertihzation (see table 6). 



These results show that the membrane is more easily ruptured 

 soon after fertilization than later, i.e., its consistency and ten- 



^° J. Loeb, Arch. f. Entwicklungsmech., 1895, vol. 1, p. 453. 



