THE MORPHOLOGY OF FERTILIZATION II 



fertilization membrane is smooth and spherical again. The 

 phenomenon is observed again just before the first division. There 

 is Httle doubt that these changes in the structure of the fertilization 

 membrane are caused or triggered off by the diffusion of certain 

 substances from the cortex into the perivitelline space. It is an 

 open question whether the fertilization membrane has the power 

 of self-propagation once it has been activated, or whether the 

 propagation of the effect is due to the progressive diffusion of a 

 substance out of the cortex, the escaping substance simply causing 

 a localised expansion of the membrane. 



The mechanism responsible for the elevation, as opposed to the 

 formation, of the fertilization membrane is not clear, though it has 

 always been assumed that release of substances between the 

 vitelline and plasma membranes, to which the former is imperme- 

 able, causes an influx of water with consequent formation of the 

 perivitelline space. The escaping (and exploding?) cortical 

 granules could conceivably be responsible for this allegedly 

 osmotic phenomenon. Apart from these considerations, the con- 

 tents of the perivitelline space, in which the concentration of 

 solids is about 0-07% (Mitchison & Swann, 1953), require further 

 investigation, as in spite of the observations of Gray (1927) and 

 R. Chambers (1942), which suggest that the perivitelline fluid is 

 liquid, Hiramoto (1954) has recently claimed, as Fol did in 1879, 

 that it has viscous-elastic properties which can be removed by 

 treatment with calcium-free sea water. 



Structures somewhat similar to the fertilization membranes of 

 echinoderm eggs are found in other eggs, for example in those of 

 Branchiostoma, prototherian mammals, Ascaris, the frog and trout. 

 In the frog's egg the membrane in question is called the vitelline 

 membrane and in that of the trout, the chorion. In both cases the 

 membrane separates or peels off from the egg surface, after ferti- 

 lization; but the same thing happens, though much more slowly, 

 in the frog's egg, if the unfertilized egg is left in tap water, from 

 which one might conclude that these eggs sustain an abortive 

 parthenogenetic stimulus by immersion in fresh water. Bialas- 

 zewicz (19 1 2) produced rather convincing evidence that the growth 

 of the perivitelline space in frogs' eggs was an osmotic phenomenon 

 associated with the presence of proteins, derived from the egg, in 

 the perivitelline space. 



There is no phenomenon comparable to the elevation of the 



