Different Types of Sex Determination 463 



individual. This again may show any number of variants. I drew at- 

 tention in my books (1920/?, 1931) to such cases as the trematode 

 Wedlia, where, according to Odhner, one egg divides and the two 

 daughter cells of the same genetic, hermaphroditic constitution be- 

 come encysted. One develops into a small functional male with rudi- 

 mentary female organs; the other, into a large female with rudimen- 

 tary male organs. The internal environment (e.g., growth substances, 

 oxidation systems, and the like) thus affect developmental features 

 within the same genetic system. Mutatis mutandis, this is exactly the 

 same as if in a hermaphrodite maize plant lateral shoots had female 

 flowers; and terminal ones, male flowers, that is, hermaphroditic 

 flowers with suppression of one set of anlagen. In plants we know 

 genetic modifiers which can affect primary differentiation, producing 

 superimposed intersexuality. In trematodes no intermediate steps are 

 known; only the final one, development of a 2X-1X mechanism {Schis- 

 tosomum: Lindner, 1914). 



In view of these facts it is not surprising that in animal monoecists 

 also, all types and grades of modification, on the same genetic basis, 

 are found, produced not only by all kinds of norms of reaction but 

 also by genetic modifiers in cases where only one definite effect oc- 

 curs (e.g., obligatory protandric hermaphroditism). Certainly no 

 borderline can be established between simple norm of reaction and 

 genetic modifiers. The details of these modifying conditions in differ- 

 ent monoecists (e.g., control by position in body, by age of individual, 

 by chemicals and hormone-like substances, and by genetically con- 

 trolled cycles) are very interesting. Many types are discussed and 

 classified in my books (1920Z?, 1931), and new ones have been added 

 especially by Coe for mollusks and Hartmann and students for worms. 

 But nothing has changed the fact that we can differentiate only be- 

 tween monoecists with F/M in equilibrium and dioecists with a 2X-1X 

 mechanism for controlling two equilibria, though transitions between 

 the two (Amphibia) are not missing. All this shows that the idea of 

 phenotypic sex determination is not only vague but actually harmful, 

 because confusing. 



The correctness of this conclusion may be witnessed even by work 

 from the Hartmann school. C. Hauenschild (1953) studied sex in a 

 polychaete annelid, Grubea clavata. The main facts (which may be 

 taken as a model for many more or less similar ones) are these. In 

 different cultures of the worm 0-69 per cent of females are found, but 

 never hermaphrodites. The males always remain males, but 37 per cent 

 of the females, after laying eggs, transform into males and remain so. 



