GENERATION AND DEVELOPMENT 713 



in this way, through the endothelial lining of the vessels of 

 mother and embryo, the blood of the foetus receives oxygen and 

 gets rid of carbon dioxide. 



Umbilical Cord. — After the formation of the foetal envelopes 

 the body walls rapidly close in, the splanchnopleure being re- 

 ceived up into the body to form the primitive gut and its deriva- 

 tives, the somatopleure forming the body wall and the limbs. 

 The embryo or foetus maintains its connections with the placenta 

 by means of the umbilical cord. This is composed of structures 

 connected with the amnion and the body wall at the umbilicus ; 

 of others in connection with the allantois and the urachus, and 

 with these the umbilical arteries and vein, or veins (ruminants). 

 All are cemented together by an embryonic connective tissue, 

 the Whartonian jelly. 



Determination of Sex. — Heape* maintains that there is no such 

 thing as a pure male or female animal, but that all contain a dominant 

 and a recessive sex, excepting hermaphrodites, in which both sexes 

 are equally represented. The assumption of male characteristics 

 in old females, and of female characteristics in old males, of the 

 human species is noted by Heape. We have referred on p. 692 to 

 the remarkable effect of castration and ovariectomy on the skull of 

 young cats, castration producing a female skull, ovariectomy a skull 

 of the male type. Caponing also induces female plumage. Injuries 

 of the ovaries in birds, as previously stated, are associated with 

 crowing and male plumage — all of which is evidence that the recessive 

 sex asserts itself when the dominant sex becomes impaired, and 

 supports the view held by Heape and others that there is no such 

 thing as a pure male or female animal. If yiis be true, it naturally 

 follows that a male ovum is fertilised by a female spermatozoon, 

 and a female ovum by a male spermatozoon (Heape). Everything, 

 in fact, points to ova and spermatozoa being sexual — that is to say, 

 there are male and female ova, male and female spermatozoa. 

 Microscopic differences in the structure of spermatozoa have also 

 been observed, which have led to their classification into two groups, 

 which are, in all probability, male and female. 



The bearing of Heape's work on the determination of sex is of 

 great importance. He maintains that the sex of the offspring is 

 fixed at the time of fertilisation, and that no influence exerted 

 subsequently can alter it. This is opposed to popular ideas, but 

 results from an acceptance of the hypothesis that an ovum in which 

 one sex is dominant must be fertilised by a spermatozoon in which 

 the opposite sex is dominant ; whether the sex be determined by the 

 ovum or spermatozoon depends upon which is the more powerful 

 of the two. Heape's study of the ovary of the rabbitf shows that 

 ova may degenerate, and that one of the chief causes is nutrition. 

 He is of opinion that nutrition has a selective action on ovarian 

 ova, and so effects a variation in the proportion of the sexes of the 



* ' Notes on the Proportion of Sexes in Dogs,' by W. Heape, M.A., 

 F.R.S., Proceedings of the Cambridge Philosophical Society, vol. xiv., 

 part, ii., 1907. 



f ' Ovulation and Degeneration of Ova in the Rabbit,' Proceedings of 

 the Royal Society, B, vol., lxxvi. 1905. 



