No. 581] ENTRANCE OF THE SPERMATOZOON 269 



of purpuratus are immobilized by NaCX no egg of the 

 same species can be fertilized, no matter how concen- 

 trated the sperm; while the same sperm when it revives 

 from the effect of NaCN fertilizes the same eggs at 

 once. This meets with the possible objection that the 

 motility of the sperm might be only necessary to allow 

 the latter to penetrate the jelly surrounding the egg 

 protoplasm. In order to test this objection the writer 

 freed the eggs of purpuratus from this jelly by treating 

 them for two minutes in a mixture of 50 c.c. sea water 

 + 3 c.c. N/10 HG1 in which all the jelly is dissolved. The 

 eggs were washed afterwards in sea water and it was 

 found that if sperm was added practically all were fertil- 

 ized. The writer put such eggs with sperm which was 

 immobilized by NaCN. The eggs and the sperm were 

 squirted together with a pipette in order to bring about a 

 close contact. No matter how concentrated the sperm 

 was, not a single egg was ever fertilized. As soon as 

 the spermatozoa recovered and showed only a slight 

 degree of motility fertilization became possible. This 

 leaves no doubt that the motility of the sperm is one 

 of the forces required to bring the spermatozoon into 

 the egg. 



That motility is not the only force was already indi- 

 cated by the previous chapter which made it clear that 

 even if the sperm is active it can not enter the egg unless 

 certain physical conditions at the phase boundaries of 

 egg, spermatozoon and surrounding solution were right. 

 In order to leave no doubt about this fact the following 

 experiments may be quoted. If we put NaCl sperm 14 

 of purpuratus or of Arbacia into a neutral mixture of 

 NaCl-f KC1 containing eggs of the same species the 

 sperm will sooner or later become very active. Yet not 

 a single egg is fertilized. If we make the solution 

 slightly alkaline the sperm becomes at once extremely 

 active yet with a few exceptions no egg is fertilized ; while 

 much less active sperm will fertilize all the eggs if CaCl 2 

 is added. The second fact is this : that the most active 



