4 FERTILIZATION 



the behaviour of eggs in one class when those in other classes be- 

 have differently. There is, admittedly, a tendency for the sperm 

 nucleus to remain relatively quiescent within the egg until after 

 the formation of the second polar body; but this is not a sufficient 

 reason for ignoring the fact that in the majority of phyla, fertili- 

 zation does not occur at the same time in the life-history of the 

 unfertilized egg as it does in sea-urchins. We shall return to this 

 question when considering the metabolism of fertilized and un- 

 fertilized eggs. 



Jelly and membranes. In some cases, there are what appear to be 

 barriers between the egg and the spermatozoon. In echinoderms 

 and frogs, for example, this barrier takes the form of a gelatinous 

 shell round the unfertilized egg, through which the spermatozoon 

 must bore or burrow to reach the egg surface. The egg of the 

 salmon is surrounded by a rather tough chorion, which is im- 

 permeable to spermatozoa except at one point, the micropyle; this 

 is a narrow channel in the chorion, through which spermatozoa 

 must pass to reach the egg. When an unfertilized salmon egg is 

 put into fresh water, the chorion hardens, the micropyle becomes 

 occluded and the egg is unfertilizable. This is one of the reasons 

 why breeders of trout and salmon mix eggs and spermatozoa 'dry', 

 before dilution with fresh water, though not all of them realise 

 that the success of 'dry' insemination is due to the egg micropyles 

 remaining open in the presence of seminal plasma as opposed to 

 fresh water. The other reason for mixing salmon or trout eggs with 

 their respective semen in the 'dry' condition is because the sperma- 

 tozoa of these fish only live for a few minutes after dilution with 

 fresh water. 



Many insect eggs are surrounded by hard and, one would have 

 thought, impermeable egg shells, which contain several so-called 

 micropyles. Insect spermatozoa, however, do not necessarily make 

 use of these structures, which are often too small for the passage 

 of a spermatozoon, and in many species, fertilization occurs before 

 the egg shell is laid down. Insect spermatozoa are sometimes said 

 to contain enzymes capable of dissolving or softening up egg shells. 

 They can penetrate thin wax layers round the egg (Beament, 1946) ; 

 but a careful perusal of Cragg's interesting paper (1920) on copula- 

 tion in Cimex lectiilarius Linn, shows that the claim that bed bug 

 spermatozoa can 'burrow' through chitin is less certain than has 

 sometimes been thought. E. B. Wilson (1928) noticed that the 



