THE FERTILISATION OF THE EGG 171 



initiates development without materially contributing to the 

 structural characteristics of the individual so formed, it is 

 legitimate to treat these two issues quite independently. The 

 nature of the hereditary process will be considered later. 



The immediate problem of fertilisation has another aspect 

 besides the elucidation of the mechanism by which the cleavage 

 process is brought into operation, namely what factors operate 

 to bring about contact between the sperm and the egg. 

 Logically perhaps it would be better to consider, first of all, 

 the attraction (if any) of the egg for the sperm ; but since our 

 knowledge of the fertilisation process is largely derived from 

 elimination of the sperm by the use of physico-chemical 

 reagents, it is just as convenient to begin with the mechanism 

 which initiates cleavage. 



Parthenogenesis exists as a normal occurrence in nature 

 in many groups of the animal kingdom, though authentic 

 cases in Vertebrates (observed with experimental safeguards) 

 are not known. In some species of stick-insects and gall-flies 

 the male has been eliminated. The existence of natural 

 parthenogenesis has prompted many biologists to imitate the 

 operations by which the agency of the sperm can be dispensed 

 with in nature. The first fruitful work in this field was done 

 by Loeb (1899), whose labours have enriched so many and 

 diverse branches of general physiology. Loeb, who con- 

 temporaneously with Ringer was a pioneer in studying the 

 relation of contractile tissues to electrolytes, was impressed 

 with the fact that stale eggs of marine animals sometimes 

 show signs of cleavage in process of dissolution, and began 

 his researches in the endeavour to explore the possibility 

 of producing artificial fertilisation by an increase in the 

 hydrogen-ion concentration of the sea water. This was not 

 in the first place successful. The action of other ions was then 

 investigated, and successful rearing of swimming pluteus 

 larvae (a stage which is taken as indicating completely successful 

 development, since the pluteus is self-supporting) from 

 uncontaminated eggs of the sea-urchin Arbacia was obtained 

 by exposing the eggs for a certain period to a mixture formed 

 by adding a hypertonic solution of magnesium chloride to 



