176 COMPARATIVE PHYSIOLOGY 



the egg-membrane. That is to say, the increase begins when 

 the sperm is still only in contact with the outside of the egg ; 

 and the curve for rate of oxygen consumption (and CO2 

 production) is steepest during the phase of surface contact. 



That the sperm brings about profound changes w^hile still 

 in surface contact with the egg is shown by LiUie's experiments 

 on eggs of Nereis, in which the sperm does not penetrate 

 the cytoplasm till thirty minutes after the initial phase of 

 fertilisation. Meiosis is initiated by the surface contact 

 of the sperm ; but if the jelly surrounding the egg is separated 

 from the latter, taking with it the sperm itself, meiosis is not 

 followed as in the ordinary course of events by cleavage. 

 The chromosomes break down without the formation of the 

 first cleavage-spindle derived from the nuclear apparatus of 

 the egg. 



Turning now to another side of the problem of fertilisation, 

 there is no need to emphasise the fact that the spermatozoa of 

 practically all animals (and many plants) are flagellate units. 

 We have, therefore, to inquire how the motility of the sperm 

 is so regulated that it is brought into contact with the egg of 

 the same species. In introducing this question it is necessary 

 to refer to the normal behaviour of spermatozoa. Spermatozoa 

 are almost without exception immobile while they remain in 

 the gonad or generative duct of the male. They usually 

 become active in the medium in which fertiHsation occurs. 

 Generally in marine animals this is the sea ; but in some 

 starfishes the sperms do not become very active in sea water, 

 unless its hydroxyl-ion concentration is raised, or egg secretions 

 are added. In mammals the sperm becomes motile in the 

 secretion of the accessory glands (prostate, etc.) ; but sperm 

 taken from the epididymis becomes active in Ringer's solution. 

 The sperm appears to possess no means of taking in nourish- 

 ment — at least in marine forms, though it may be able to do so 

 in animals such as bees and bats, in which insemination may 

 take place months or even years before fertilisation ; and it 

 therefore has a strictly Hmited term of Hfe. Cohn (19 18) 

 has shown that the total carbon dioxide output of the sperm 

 is the same whether its life is artificially prolonged or curtailed 



