[189] Correns, C., _Biol. Centralbl._, 1916, xxxvi., 12. _II. The Physiological Basis of Sex Determination_ 5. As stated at the beginning of this chapter, the chromosome theory of sex determination explained only one feature of the problem, namely, the relative numbers in which both sexes or only one sex, as the case may be, are produced; and in this respect the evidence is so complete that we must accept it. But with all this, the problem of sex determination is not exhausted, since a physiological solution of the problem of sex determination demands an account of how the sex chromosomes can induce the formation not only of ovaries and testes but also of the other sex characters. For the solution of this problem biology will have to depend largely on experiments in which it is possible to influence the formation of sex characters and of the sex glands themselves. The most striking observations in this direction were made by Baltzer on a marine worm, _Bonellia_. In this animal the two sexes are very different, the male being a tiny parasite, a few millimetres in length, which spends its life in the uterus of the female, whose size is about five centimetres. A female carries as a rule several and often a large number of the male parasites in its uterus, which indicates that the males prevail numerically. The fertilized eggs of the animals are laid in the sea water where the larvæ hatch. At the time of hatching all larvæ are alike. The differentiation of the larvæ into the dwarf males and the giant females can be determined at will. The larvæ have a tendency to attach themselves to the proboscis of the female as soon as they hatch. If given a chance to do so and if they stick to the proboscis for more than three days they will develop into males, which soon afterwards creep into the female where they continue their parasitic existence. If, however, no adult female _Bonellia_ is put into the aquarium in which the larvæ hatch, about ninety per cent. of the larvæ will, after a period of rest, develop into females; the rest develop into males. Those which develop into females will often show a primary maleness which may manifest itself in the production of sperm or of other secondary male sexual characters. This tendency is stronger the longer the period of rest lasts. If the larvæ are allowed to settle on the proboscis of the adult female but are removed too early hermaphrodites are produced having male and female characters mixed.