22 EMBRYOLOGY. 



For the purpose of locomotion and in order to make possible the 

 union with the non-motile egg-cell, it has become metamorphosed 

 into a contractile filament, and has rid itself completely of all 

 substances, as, for example, yolk-material, which would interfere 

 with this principal requirement. At the same time it has assumed 

 the form best adapted for passing through the envelopes with 

 which, as a means of protection, the egg is surrounded, and for 

 penetrating the yolk. 



The conditions especially in the vegetable kingdom confirm the 

 accuracy of this interpretation. There are plants of the lowest 

 form* in which the two copulating sexual cells are entirely alike, 

 both being small and motile ; and there are other related species in 

 which a gradual differentiation is brought about by the fact that 

 one of the cells becomes richer in yolk and incapable of motion, 

 while the other becomes smaller and more active. From this it is- 

 \i<lent that the stationary egg must now be sought out by the 

 migratory cell. 



A few physiological statements may be in place in this connection. 



In comparison with other cells of the animal body, and especially 



in comparison with the eggs, the seminal filaments are characterised 



by greater duration of life and power of resistance, a fact which is 



1 1 -fluently of importance for the success of fertilisation. The mature 



spermatozoa, after they are set free from their connection with 



r cells, remain for months in the testes and vasa deferent ia 



without losing their fertilising power. They also appear to remain 



-o for a long time after having been introduced into the sexual 



passages of the female, perhaps for several weeks in the case of Man. 



in-- animals this is demonstrable to a certainty. For example, it 



is known that the semen of Bats remains alive in the uterus of the 



femnl*' during the whole winter; and in the case of the Fowl it is 



kn<<\ rtilised eggs can be laid up to the eighteenth day a: 



the removal of the Cock. 



In the presence of external influences semen shows itself to be 

 much more resistent than the egg-cell, which i> i-ily iniur <1 or 

 kill.il. For example, when semen is frozen an<l th.-n thawed out, 

 motion of the seminal filaments comes back again. Many s: 

 *-y are employed not too strong, have no del. 



Narcotics in strong concentration, and when employ. .1 tor a long time, 

 make the filaments motionless, without immediately killing them, 

 because after removal of the injurious substance they can be revi 



