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 

 forms 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 

 evident 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 

 frequently of importance for the success of fertilisation. The mature 

 spermatozoa, after they are set free from their connection with 

 other cells, remain for months in the testes and vasa deferentia 

 without losing their fertilising power. They also appear to remain 

 active for a long time after having been introduced into the sexual 

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

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

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

 female during the whole winter ; and in the case of the Fowl it is 

 known that fertilised eggs can be laid up to the eighteenth day after 

 the removal of the Cock. 



In the presence of external influences semen shows itself to be 

 much more resistent than the egg-cell, which is easily injured or 

 killed. For example, when semen is frozen and then thawed out, 

 the motion of the seminal filaments comes back again. Many salts, 

 if they are employed not too strong, have no deleterious influence. 

 Narcotics in strong concentration, and when employed for a long time, 

 make the filaments motionless, without immediately killing them, 

 because after removal of the injurious substance they can be revived. 



