194 THE PHYSIOLOGY OF REPRODUCTION 



be observed on the manometer scale. If we take the reading on 

 the unfertilised eggs at the end of ten minutes, we can probably 

 approximate to it roughly by dividing this figure by 10. It works 

 out for a number of experiments at something like 0-f5 cub. mm. 

 of oxygen per minute. The same eggs fertilised consume in the 

 first minute after the addition of the sperm 12 cub. mm. of oxygen. 

 Thus the addition of the sperm to the eggs causes, within the space 

 of one minute, an increase in their oxygen consumption of something 

 like eighty times that observed on the same lot of • eggs one minute 

 previous to the addition o£ the sperm. 



The examination of sections of fixed material of these fertilised 

 eggs shows that the process is by no means an instantaneous one, 

 and that the sperm take ten to fifteen minutes before they find 

 their way into the actual cytoplasm of the eggs. 



This initial oxygen consumption of the egg immediately on 

 fertilisation must be induced by the first contact of the sperm 

 with the external surface of the egg-membrane. We arrive then 

 at the remarkable conclusion that mere contact of the spermatozoon 

 with the external surface of the egg-membrane is capable of 

 increasing the oxygen consumption of the egg by something like 

 8,000 p6r cent, in the course of one minute. 



There are good reasons for believing, as the result of Loeb's 

 experiments on the fertilisation of the eggs of Strongloeentrotus with 

 the sperm of Asienas, and Lillie's description of the process of 

 fertilisation in Nereis, that the entry of the spermatozoon into the 

 egg consists of two distinct phases. First, an external one, in which 

 certain changes are brought aboi^t in the cortical substance, of the egg 

 the moment the sperm make contact with the external surface of the 

 egg-membrane. This would seem to be correlated with this' initial 

 oxidation taking place in the egg as described above for E. viicro- 

 tuberculatus. Secondly, the changes following the actual entry of the 

 spermatozoon into the egg cytoplasm itself, which, as Lillie has shown 

 in Nereis, only takes place some thirty minutes af iter the first phase 

 of fertilisation, and in the sea-urchin, follows some ten to fifteen 

 minutes after the sperm are added to the eggs. 



By centrifuging the eggs of Nereis before the sperm has actually 

 penetrated the egg-membrane, Lillie was able to separate the jelly 

 surrounding the egg and containing the spermatozoon from the egg 

 itself. These eggs complete meiosis which has been initiated' by the 

 spermatozoon, but never segment. A typical segmentation nucleus 

 is, however, formed, which breaks down, leaving the chromosomes 

 free in the egg cytoplasm; they split .longitudinally in the normal 

 manner but never separate. No asters or mitotic spindles appear in 

 these eggs, as when the complete process of fertilisation is .allowed to 



