188 PHYSICAL BASIS OF HEREDITY 



Sex-determination and Abtificial Parthenogenesis 



Many interesting questions concerning sex-detennina- 

 tion might be studied were it as easy for man, as it appears 

 to be for nature, to make eggs develop without fertilisa- 

 tion. Only three cases are known in which eggs developing 

 under artificially induced conditions have reached matur- 

 ity. Delage raised one sea urchin that had been produced 

 artificisilly to maturity, and determined that it was a male. 

 Tennent has shown that the male is heterozygous for the 

 sex-chromosomes. Hence, if the artificially produced 

 urchin has the half number of chromosomes it should, if 

 like the bee, be a male, but if, as Herlandt has shown, the 

 number of chromosomes may double before development, 

 a female would be expected. 



In the frog, Hertwig, and later his pupil Kuschake- 

 witch, found that the number of males is increased up to 

 100 per cent, if the eggs are detained in the uterus for 

 one to three days before adding sperm to them. Hertwig 

 has attempted to explain the result as due to a relative 

 change in the size of the nucleus that takes place in conse- 

 quence of the delay, but since the chromosomes are at this 

 time in the metaphase of the second polar spindle, it is not 

 obvious how such an enlargement could be brought about, 

 quite aside from the question as to whether the result 

 imagined would follow even after such a change. I have 

 suggested that these eggs with deferred fertilization may 

 develop parthenogenetically, due either to the egg nucleus 

 alone giving rise to the nuclei of the embryo, or to the 

 sperm alone giving rise to these nuclei, in the latter case, 

 the polar spindle of the egg having been caught at the sur- 

 face and prevented from taking part in the development. 

 The possibihty of the nuclei of the frog arising in one 

 or the other of these ways is shown by the work of Oscar 

 and Gunther Hertwig who have found evidence that after 

 treatment with radium, the sperm-nucleus alone may give 

 rise to the somatic nuclei of the embryo. Packard also 



