THE PHYSIOLOGY OF FERTHJZATION 139 



the agglutinablc substance of the spermatozoon, which 

 is apparently lost by staling, as we have previously 

 seen. 



The main principle of this discussion, viz., that 

 spermatozoa may lose their fertilizing power for other 

 causes than loss of motility, or that motility alone is 

 not an adequate criterion of fertilizing power of sper- 

 matozoa, has obvious important bearings. The mere 

 fact that spermatozoa may retain their motility for 

 three weeks or more in the human genital tract (Wal- 

 deyer, 1906) by no means proves that they retain their 

 fertiHzing power during all this time, although this has 

 been almost universally assumed. In an excellent paper 

 published after his death. Mall (1918) points out the 

 many contradictions and unnecessary assumptions that 

 this belief entails with reference to the facts of human 

 conception, and he concludes that it is probable that 

 spermatozoa have lost their fertilizing power by the 

 time they have passed the tube. Bryce and Teacher 

 (1908) and Triepel (1914-15) also conclude that ferti- 

 lization must occur within forty-eight hours after 

 copulation. 



2. The ovum. — Ova attain a fertilizable condition 

 rather suddenly, as a rule at the very end of the period 

 of growth or at the beginning of the maturation period. 

 The egg of Nereis is fertilizable before the rupture 

 of the germinal vesicle, but the ova of sea urchins, 

 starfish, Dentalium (mollusk), and Nemerteans are 

 not fertilizable until the germinal vesicle has begun 

 to break down (see section on merogony). Ova may, 

 however, be penetrated by spermatozoa at an earlier 

 period but without any fertilization reaction occurring. 



