?38 TRANSACTIONS OF SECTION D. 



12. Rejyort on the Occu^mtion of a Table at the Marine Laboratory, 

 Plymouth. — See Reports, p. 304. 



13. Report on Experiments on the Development of the Frog, 

 See Reports, p. 304. 



14. A recent Visit to the Ceylon Pearl Banks, 

 liy Professor W. A. Herdman, F.R.S. 



The author gave some natural history notes on the life and surroundings of 

 the pearl-oysters from observations made during a recent visit to the Ceylon pearl 

 banks. These dealt with (1) the kind of ground the oysters live upon and the 

 objects to which they are attached ; (2) the oyster-eating fishes and other enemies 

 that may affect the life of the oyster; and (8) the different types of oyster that 

 occur and the question of their constancy on certain grounds. 



15. Wild Ancestors of the Domestic Hone. 

 By Professor J. Cossar Ewart, F.R.S. 



MONDAY, SEPTEMBER 7. 



Joint Discussion with Section K on Determination of Sex, 

 Opened by L. Doncaster, M.A, 



Until rather recent years there was the utmost diversity of opinion as to the 

 determination of sex. Some regarded it as depending on nutrition, others on 

 the age of the parents or maturity of the germ-cells, some as depending wholly 

 on the egg, and others, again, on the spermatozoon. Gradually, however, a cer- 

 tain amount of order has emerged from this chaos. In the first place, the facts of 

 parthenogenesis made it clear that in many cases at least the sex was determined 

 irrevocably in the Gg^ before segmentation ; and the same thing was shown by 

 such instances as Dinophilus and certain mites, in which the eggs which will yield 

 females are larger than those producing males, although both need fertilisation. 

 The bee and those animals which behave similarly, on the other hand, indicate 

 that sex may be modified by the spermatozoon, for in them virgin eggs yield 

 males, fertilised eggs females ; but here, again, no treatment after fertilisation will 

 turn a female into a male or the reverse. It may therefore be regarded as 

 established in very many cases that from the moment of fertilisation at least, and 

 sometimes in the unfertilised egg, the sex is irrevocably determined. 



The problem had reached this stage when M'Clung, Wilson, and others dis- 

 covered that in certain insects the males and females contain diflereat numbers of 

 chromosomes in the germ-cells before maturation, the females having an even 

 number and the males one less. After maturation there are two kinds of sperma- 

 tozoa, one containing the same number as the mature &Qg, and the other having 

 one chromosome missing. It was at first suggested that at fertilisation the 

 spermatozoon having the larger number caused tlie eg^r to develop into a female, 

 that with the smaller number male ; but Wilson's later suggestion is that 

 there is selective fertilisation, that the eggs are either male or female, and that 

 male eggs are fertilised by spermatozoa having no heterochromosome, female eggs 

 by those which have it. Morgan has recently found that in a species of Phyllo.vera 

 there are two kinds of spermatids, one of which has one chromosome more than 

 the other. Those with the smaller number degenerate ; those with the larger 

 develop into functional spermatozoa, and all fertilised eggs become females. 



