114 FERTILIZATION 



pretation is that butyric acid treatment by itself induces the 

 invisible change, but inhibits the cortical change. When the 

 capillary and egg were placed in sea water containing detergent, 

 a membrane appeared round the proximal, but not the distal, 

 part of the egg. The detergent induced the breakdown of cortical 

 granules without the invisible change. 



Type I Inhibition seems to involve more complicated mechan- 

 isms in mammalian eggs. When a rat spermatozoon passes through 

 the zona pellucida of a rat tgg, a minute slit or hole remains where 

 the spermatozoon penetrated (Austin, 195 1 6). Braden et al. (1954) 

 examined the relative positions of these slits in dispermic rat eggs, 

 and obtained the distribution shown in Table 18, which shows that 



TABLE 18 



Angle, subtended at the centre of a dispermic rat egg, by two sperm 

 slits in the zona pellucida {Braden et al., ig54 ) 



the most probable place for a second spermatozoon to penetrate 

 the zona is in the opposite hemisphere to that which the first 

 spermatozoon penetrated. Braden and his co-workers conclude 

 from this and other information about the number of spermatozoa 

 entering eggs and the perivitelline space that the first spermato- 

 zoon to penetrate the zona initiates a self-propagating structural 

 change, analogous to the cortical block to polyspermy, in the zona. 

 By a probabilistic analysis which is somewhat similar to that used 

 in the earlier calculations to do with the conduction time of the 

 cortical block to polyspermy, they estimate that the conduction 

 time of the change in the structure of the zona, which makes it 

 impermeable to spermatozoa (see p. 12), is between 10 and 90 

 minutes. If this interpretation of the facts is correct, mammalian 

 eggs have two blocks to polyspermy, one propagated round the 

 cortex and the other round the zo?ia pellucida. The latter appears 

 to be a rather inefficient mechanism, if the minimum conduction 



