x] COMPARISON WITH FERTILISATION 145 



supported by the experiment of fertilising Sea-urchin eggs 

 with starfish sperm (which can be done if the eggs are first 

 suitably treated); under these circumstances some of the 

 eggs form membranes but then disintegrate unless they 

 are subsequently treated with hypertonic sea-water, while 

 others continue to segment normally without the hypertonic 

 solution. This suggests that the spermatozoon introduces a 

 cytolytic agent into all the eggs, since all produce mem- 

 branes, but that the foreign sperm does not always introduce 

 the proper corrective to prevent complete cytolysis. Al- 

 though the spermatozoon appears to introduce a cytolytic 

 agent, this does not explain why nuclear and cell-division 

 are brought about by its entrance, and LOEB concludes that 

 this is caused by increased oxidation. It is known that eggs 

 immediately after fertilisation, or after treatment which 

 gives rise to artificial parthenogenesis, absorb oxygen much 

 more rapidly than unfertilised eggs, and the same is true 

 of eggs undergoing complete cytolysis; it seems probable, 

 therefore, that the cytolysis of the egg-surface which results 

 in membrane formation allows increased oxidation of the 

 egg, and that this increased absorption of oxygen is the 

 stimulus which brings about division of the nucleus followed 

 by that of the cell as a whole. 



The work of GRAY (1916) on the electrical conductivity of 

 Echinoderm eggs before and after fertilisation or treatment 

 with substances that bring about artificial parthenogenesis 

 suggests a somewhat different explanation of fertilisation 

 from that arrived at by LOEB, though it does not alter the 

 general conclusions. GRAY'S experiments indicate that both 

 fertilisation and treatment with parthenogenetic agents 

 alter the electrical condition of the egg surface, making it 

 more permeable to electrolytes. He therefore regards the 

 subsequent changes as due to an alteration of the surface 



B.C. 10 



