28 GENERAL CONCEPTIONS. 



comprehensive name of hormones (Starling). Now, it has become probable that 

 the sexual glands produce hormones, which exert an effect upon other organs; for 

 example, the mammalian corpus luteum is believed to yield a hormone, which so 

 affects the uterus as to render it adapted to pregnancy. The hypothesis accord- 

 ingly naturally suggests itself that hormones from the sexual glands occasion the 

 development of the secondary sexual characteristics. It is well to bear in mind that 

 our .knowledge of hormones is still meager, and that the suggested hypothesis may 

 or may not prove valid. 



Concerning the cause of sex, i. e., why one individual is male and another 

 female, we know very little. Nothing positive as to .the cause of sex in vertebrates 

 is known, though many speculations have been published. In certain insects, 

 however, it has been discovered that the sexes are distinguished by the females 

 having one more chromosome in each cell nucleus than the males.* This difference 

 is explained by the fact that there are two kinds of spermatozoa, one of which 

 contains an extra (accessory) chromosome. Those ova which at the time of fer- 

 tilization receive an accessory chromosome become females, those that do not, 

 males. At present we can add only that the important discoveries mentioned may 

 furnish the clue to solve the problem of the cause of sex. 



The Theory of Heredity. 



We owe to Moritz Nussbaum the theory of germinal continuity the only 

 theory of heredity which seems tenable at the present time. According to this 

 theory, the germ-cells are set aside during the segmentation of the ovum and pre- 

 serve the essentially undifferentiated qualities of the protoplasm and nucleus of the 

 ovum, from the division of which they arise. Just as the cells formed during seg 1 

 mentation are capable of producing the various tissues of the body, so the germ- 

 cells have and preserve this faculty. If we term the material of the original ovum 

 germ-plasm, we may say that this germ-plasm gives rise to the various tissue- 

 forming cells which make up the body. And by this very conversion into tissue 

 cells that germ-plasm is changed, and is no longer, as we have learned before, 

 capable of the full range of development. The germ-cells, on the contrary, do re- 

 main so capable and it is precisely in order to preserve this capacity that they hold 

 aloof from the formation of the body tissues and pursue their own independent 

 career. A portion of the germ-plasm of the parent ovum is, so to speak, short- 

 circuited into the genital elements which produce the offspring. 



If we accept this view, we are forced to make the supplementary hypothesis 

 that the conspicuous complicated changes, by which the germ-cells are converted 

 into sexual elements, do not involve differentiation in the true sense i. e., strictly " 

 comparable to that which we observe in the somatic cells. Although this hypothe- 



* In other insects more complicated relations have been discovered. The relation of the chromosomes to sex 

 appears to be a complicated and difficult problem. 



