STUDIES IN ARTIFICIAL PARTHENOGENESIS. 1 93 



may exist between norma' and treated eggs. When normal 

 unfertilized Arbacia eggs are centrifuged vigorously, the pro- 

 toplasmic materials rapidly become separated into four layers 

 or zones. Lyon ('07) has given an excellent description of the 

 appearance of these zones and the reader is referred to his paper 

 for detai s. The pigment-bearing granules come to be all massed 

 at one pole, in the pigment zone; next to this is a zone of granular 

 material, then a hyaline zone, and at the pole opposite the pig- 

 ment zone is a small dense accumulation of substance which 

 because of its color is known as the gray cap. The egg nucleus 

 lies in the hyaline zone, directly beneath the gray cap. When 

 an egg shows all these zones, I shall refer to it as "stratified." 

 As the viscosity of the protoplasm increases, stratification be- 

 comes more and more difficult; in a thoroughly coagulated egg 

 no stratification is possible. 



Hypertonic solutions produce a very noticeable coagulative 

 change in the sea-urchin egg. In 1913 a few preliminary ex- 

 periments were performed in which the eggs were pressed out 

 of shape by pushing down on the coverslip with a dissecting 

 needle. It was found that "Nad hypertonic sea-water >: 

 produced a marked increase in the viscosity of the cytoplasm. 

 Such a solution causes swelling of the vitelline membrane. This 

 swelling is apparently absent if a freshly prepared 0.49 M MgCl 2 

 solution is used. Eggs treated with this solution became very 

 much more viscous than they had been previously. Whereas 

 the normal unfertilized eggs shot out their contents rapidly if 

 subjected to a slight pressure, eggs which had been immersed in 

 MgCl 2 for 85 minutes could be subjected to considerable pres- 

 sure without losing their circular outline. They could indeed 

 be flattened out into a thin "pancake." 



More accurate data were obtained with the centrifuging 

 method. On July 23, 1914, some eggs were placed into 50 c.c. 

 of sea-water plus 8 c.c. of 2.5 M NaCl at 2:2 if P.M. At 

 2:44 P.M., these eggs were placed into one tube of the centrifuge, 

 and into the other tube were placed some normal untreated eggs 

 of the same female. At 2:45 P.M., after a few preliminary 

 turns, the tubes were revolved for 28 seconds at a rate of 162 

 revolutions per second. The eggs were then examined. The 



