EGG-VOLUME AND FERTILIZATION MEMBRANE. 55 



Of the eggs fertilized with sperm those in I to 4-cell stages had 

 diameters of egg-cytoplasm ranging from 13 to 15, and of fertili- 

 zation membrane from 14.7 to 18 mm.; those in stages from 

 8-cells to blastulse had diameters from 14.5 to 16 mm. for egg- 

 cytoplasm, and 16.5 to 17 mm. for fertilization membrane. The 

 number of eggs measured in each case was five only. But the 

 extremes of a large number as well as mean sizes were taken in 

 each case. The mean size of the hypertonic parthenogenetic 

 eggs thus appear to reach nearly the same limit that the membrane 

 does in eggs fertilized with sperm; the egg-cytoplasm of the 

 former apparently takes up water as easily as the membrane of 

 the latter. The writer's observations did not include a large 

 enough number of these eggs at the moment of membrane for- 

 mation to determine whether or not they showed a preliminary 

 shrinking, as Otto Glaser 1 and others have observed in eggs of 

 other species. One can only say that if the shrinking took place 

 it must have occurred between the moment of the entrance of the 

 spermatozoon and the moment just before the first cleavage. 



2. Asterina miniata (Brandt) Perrier. 3 When mature, and just 

 before fertilization, a sample of seven eggs of this species measured 

 19, 19, 18.5, 19, 20, 19 and 20 mm., giving a mean of 19.25 mm. and 

 an average of 19.21 mm. in diameter. When treated with suitable 

 amounts of citric acid for a while and then returned to normal 

 sea-water the eggs began to go into cleavage and develop into 

 blastulae. 4 A fertilization membrane could be seen reaching 

 across the furrows between the cells of these eggs. They differed 

 in this respect from the sand-dollar eggs that were made to go into 

 cleavage by hypertonic solutions. But nevertheless the mem- 

 branes did not free themselves from the outermost periphery of 

 the egg-cytoplasm. The diameters, therefore, of egg-cytoplasm 

 and fertilization membrane of Asterina miniata so treated under- 

 went equal changes; they ranged between 19 and 21, giving a 

 mean diameter of 20 mm. No measurements were made at the 

 moment of treatment with the acid. 



3 Thanks are due to Dr. W. K. Fisher, who kindly identified this species of 

 Asterina for the writer. 



4 The eggs of this species are known to be somewhat naturally parthenogenetic. 

 In the series with the optimum concentration of the citrir acid 79 per cent, swimming 

 larvae appeared at the end of 42 hours whereas in the control only 6 per cent, of 

 the eggs were segmented and none were swimming. The average of all the series 

 worked with showed a natural parthenogenesis to the extent of about 3.5 per cent. 



