114 THE MAMMALIAN EGG 



In the course of subsequent passage through 10 and 15 per cent 

 glycerol, however, the eggs shrank irreversibly, and from the 

 results of attempts to culture these eggs it was considered that they 

 had been killed. On the other hand, eggs treated with the same 

 concentrations of glycerol, but at 37°C and for 10 min at each 

 stage, contracted only slightly and soon re-expanded. The data 

 suggest that eggs are more permeable to glycerol at 37°C than at 

 20°C. When the eggs were freed of glycerol and placed in culture 

 in serum, most of them developed to morulae, showing that rabbit 

 eggs can tolerate exposure to relatively high concentrations of 

 glycerol at body temperature (Smith, 1952). 



Mouse eggs (unfertilized tubal oocytes) exposed to a medium 

 composed of a suspension of egg yolk in Locke's solution, to which 

 sodium citrate had been added, showed only slight shrinkage when 

 held at 5°C for up to 2 hr. If the medium contained in addition 

 glycerol at a concentration of 5 per cent, however, the eggs shrank 

 considerably and became crenated. The effect took place within 

 15 min; no re-expansion occurred in the subsequent 1 to ij hr, 

 suggesting that glycerol had not entered the vitellus during the 

 period of observation. Vitelline shrinkage evidently had little effect 

 on the viability of the eggs, for when they were transferred to 

 recipient mated mice 22-8 per cent developed normally, a propor- 

 tion that was comparable to that found with untreated eggs (Lin, 

 Sherman and Willett, 1957). 



Influence on spermatozoa. The spermatozoa of some primitive 

 plants are attracted towards the eggs by substances emanating from 

 the eggs ; this is probably best established for the ferns, in which the 

 attracting substance is L-malic acid (see Rothschild, 1956). The fern 

 spermatozoa are said to become orientated by chemotaxis, swim- 

 ming persistently towards higher concentrations of malic acid and 

 so reaching the eggs more surely than they would have otherwise. 

 Several claims have been made that a similar mechanism exists in 

 the animal kingdom, but so far they have not received general 

 acceptance. The main reason for this is the difficulty of distinguish- 

 ing between an attractive effect and a trapping action, these two 

 influences being likely to have very similar consequences in the 

 distribution and behaviour of the spermatozoa. Thus, in one in- 

 vestigation, the concentration of mouse spermatozoa was found to 

 be much higher in the region of cumulus oophorus immediately 

 surrounding the eggs than in peripheral parts of the cumulus ; but a 



