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Notes on the Proportion of the Sexes in Dogs. By Walter 
Heape, M.A., F.R.S., Trinity College, Cambridge. 
[Deceived 20 February, 1907.] 
Introduction. 
In the following paper I have confined myself to the facts 
presented by the records I have collected. 
I do not propose here to enter into a detailed argument 
regarding the causes which influence the proportion of the sexes 
produced by various animals, but would preface my remarks 
with a very brief general statement of certain aspects of that 
problem, as they appear to me. 
For generations it has been believed that the sex of an 
embryo is determined by extraneous forces exerted during the 
development of the embryo. 
The subject has always excited great interest among breeders 
and there is a huge literature dealing with it. I have myself 
noted titles of over 600 papers and books in which a great 
variety of causes have been urged as influencing the sex of the 
offspring and numerous theories published on methods to be 
adopted in order to regulate the proportion of the sexes which 
are born. 
A very large section of this mass of literature has been 
written on the assumption that sex is determined during em- 
bryonic life, that it can be and is normally determined in 
accordance with conditions which affect the growing embryo. 
But it seems clear, as I will show below, that both the ovum 
and the spermatozoan are themselves sexual, that the latest 
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