12 FERTILIZATION 



fertilization membrane in mammalian eggs; these eggs are, how- 

 ever, surrounded by a membrane, the zona pellucida, whose 

 structure changes shortly after fertilization (Smithberg, 1953; 

 Braden et al., 1954), in a way which is somewhat reminiscent of 

 the 'tanning' of the fertilization membrane. As a result of this 

 change, the zofta pellucida becomes less permeable to spermatozoa. 

 Braden et al. (1954) believe that the change in the permeability of 

 the zona pellucida to supernumerary spermatozoa is a reaction 

 propagated within the zona; but their experiments do not entirely 

 rule out an alternative hypothesis, that a substance progressively 

 released from the egg surface into the perivitelline space is re- 

 sponsible for the 'tanning' of the zona (see chapter 9). 



Entrance of sperm tail and middle-piece. In Anthocidaris crassis- 

 pina (A. Agassiz) the sperm tail lies motionless outside the ferti- 

 lization membrane for about 2| minutes after fertilization. It then 

 straightens out, radially to the egg surface, and starts moving 

 vigorously. According to J. C. Dan (1950a), this movement makes 

 the sperm tail enter the egg in 15-30 seconds. There seems to be 

 no fixed rule as to whether the tail and middle-piece of a sperma- 

 tozoon enter the egg at fertilization. In Nereis succinea, neither 

 the tail nor the middle-piece go in with the head (F. R. Lillie, 19 126), 

 while in Nyctalus noctula (Schreber), the whole spermatozoon is 

 found in the cytoplasm (van der Stricht, 1902). In spite of the 

 classical and painstaking research of Meves (191 2) on the fate of 

 the middle-piece in those cases where it does enter the egg, and 

 in spite of the fact that the whole spermatozoon often (or perhaps 

 always) enters the mammalian egg (Blandau & Odor, 1952; 

 Austin, 1953; Shettles, 1954), the existing evidence does not per- 

 mit the conclusion that either the middle-piece or the tail of the 

 spermatozoon has an important function in fertilization, after 

 attachment of the fertilizing spermatozoon. This is to be expected 

 in the case of the sperm tail, which is obviously an organ of locomo- 

 tion, at any rate in animal spermatozoa.* Recent studies suggest 

 that the middle-piece may contain mitochondria-like material 

 concerned with locomotion, and endogenous substrates (Rothschild 

 & Cleland, 1952). Austin would probably not agree with these 

 views, as he has recently said (1953, p. 196), in regard to fertilization 



* It may be that some of the tails of plant spermatozoa have a sensorj' function, 

 apart from being locomotor organs; but there is no evidence to support this idea 

 at present. 



