THE GENITAL GLANDS 19 



chemical factor, at any rate in sea-urchins and star- 

 fish, because in these animals the purely female ovum 

 can be induced to develop into a larva by concentrated 

 seawater, tannin, or even violent shaking, ^^erhaps, 

 however, these animals are not far removed from 

 parthenogenesis, and the part played by the male 

 in vertebrates is probably more important. 



After fertihzation, the ovum starts to divide into 

 two, four, eight, and so on. Much hght is thrown 

 upon the process by the phenomenon of identical 

 twins. Ordinary twins, due to the fertihzation of 

 two ova by two spermatozoa, are no more aHke than 

 any other pair of brothers or sisters. Identical twins 

 probably result from the accidental separation of the 

 two cells produced from the first division of a fertihz- 

 ation ovum, and the children have an identical 

 heredity. They are exactly ahke in sex, appearance, 

 and even in deformities such as hernia. This shows 

 that the sex and general conformation of the child 

 are probably fixed from the moment when a particular 

 ovum and a particular spermatozoon meet. 



CHEMICAL DIAGNOSIS OF PREGNANCY. 



When an unusual protein passes repeatedly into 

 the circulation, antibodies of a ferment nature are 

 produced to destroy it. Some protein from the 

 placenta passes into the maternal blood-stream 

 during pregnancy. Abderhalden has based upon 

 this a method of serum diagnosis. Fresh placenta 

 is treated with the patient's serum, and if she is 

 pregnant peptones are formed by digestion. These 

 can be dialysed off through an animal membrane. 



