133 



A. P. MAT HEWS. 



The sections, the living material and the observations on 

 Astcrias indicate that the period of great susceptibility is immedi- 

 ately before and during segmentation and that just after segmen- 

 tation great resistance prevails. 



In the following table I have placed in parallel columns one 

 of Lyon's results, and the phenomena as shown in sections of 

 the eggs. There is one objection to comparing different series 

 of eggs in that the temperature may not have been the same in 

 the different experiments, and the periods will not exactly coin- 

 cide. However the eggs of Arbacia develop very uniformly and 

 nearly always the first cleavage comes in between fifty and sixty- 

 five minutes after fertilization. 



TABLE I. 



Minutes After 

 Fertilization. 



Susceptibility to Cyanide 



Phenomena as Shown in Sections 

 and Living Material 



O 

 O-IO 



10-25 



30-48 

 48-60 



60-65 



65-70 



Slight. 



Slight increase in susceptibility. 



First period of susceptibility. 



Progressive increase in immu- 

 nity. At end most immune. 



Second period of susceptibility. 



Susceptible. 

 Immune. 



Egg at rest. 



Sperm penetrating egg. Aster 

 very small. Meets egg nucleus in 

 8-12 minutes. 



Fusion of nuclei. Great growth 

 of aster. Caterpillar stage. Rays 

 throughout cell. 



Pause stage and retrogression of 

 aster. Large rays fade out. Enor- 

 mous growth of nucleus. 



Nuclear wall fades about 45-5 

 minutes after fertilization. Tre- 

 mendous growth of asters follows 

 At 60 minutes chromosomes be- 

 ginning to separate. 



Segmentation completed. 



Retrogression of asters. 



It will be seen from Lyon's experiments that as a rule his 

 period of susceptibility came about fifty-five minutes after fertili- 

 zation or even five minutes earlier. This period is certainly just 

 before segmentation. 



It is clear from this table that the period of great susceptibility 

 coincides with the development of the asters ; and the period of 

 greatest immunity coincides with the retrogression of the asters 

 and the development of the nucleus. Thus the first decrease in 

 immunity coincides with the time when the sperm aster is devel- 

 oping. This reaches its maximum about 20-25 minutes after 



