SECTIONAL TRANSACTIONS— D. 363 



for the species could usually survive and multiply at the same rate with 

 far fewer males. Except where the male protects territory or incubates 

 or guards or feeds the female or young, its significance for the species is 

 simply to act as a dice-box for the production of new combinations of genes. 



Dr. W. O. Kermack. — Secular trends in the sex ratio and some related 

 topics {11.25). 



Dr. A. Walton. — The experimental control of the sex ratio (11.55). 



Three possible methods by which the sex of animals may be controlled 

 or the numerical proportion of the sexes altered by wilful manipulation are 

 discussed. The first method is the induction of parthenogenesis of the egg. 

 This occurs normally in many invertebrates and is a common mode of repro- 

 duction in some insects. In the vertebrates parthenogenesis has been 

 induced in amphibia and to some extent in mammalia. The second method 

 is the induction of sex reversal during development or in adult life. This 

 also occurs normally in some invertebrates and abnormally or by experi- 

 mental treatment in vertebrates. The third method is the separation of 

 the male and female determining sperms and is applicable to animals 

 including mammalia, in which the male is heterogametic. There is evidence 

 that such separation does occur normally to some extent. The results 

 obtained by experiment are as yet inconclusive, nevertheless the com- 

 parative ease with which the sperms can be subjected to experimental 

 manipulation opens up a promising line of future investigation and the 

 prospect of practical sex control. 



Dr. P. C. Roller. — -The differentiation of the sex-determining 

 mechanism (12.15). 



The diploid generation in higher animals and plants is sexually differ- 

 entiated : one sex is heterozygous with an XY pair of chromosomes, while 

 the other is homozygous having a pair of similar chromosomes, XX. There 

 are various types of differences between the X and Y chromosomes which 

 may be considered as evolutionary stages in the differentiation of the sex- 

 determining mechanism. In higher organisms the sex chromosomes are 

 structurally differentiated into a pairing and non-pairing or differential 

 segment. The heterozygous condition of one sex is maintained by the 

 suppression of crossing-over between those regions of the sex chromosomes 

 which represent the genotypic differences in the sex-determining mechanism, 

 the differential segments. The obligatory segregation of these segments is 

 ensured by the homologous region in the X and Y chromosome. 



Dr. E. B. Ford, Dr. C. Gordon, Mr. and Mrs. Culwick. 



Afternoon. 



Mr. C. H. Roberts. — The effect upon fish of rain washings from tarred 

 roads (2.15). 



The deleterious effects of rain washings from roads are due to both 

 physical and chemical factors. 



There is an element of risk with any waterproof road surface whether 

 this be of tar, bitumen or cement. When rain falls upon a waterproof road 

 it is shed directly into the nearest stream, carrying with it dust, dung, fallen 

 leaves and lubricating oil. The organic matter decomposes and acts in 



