4. While thus far all the facts agree with the dominating influence of certain chromosomes upon sex determination, one group of facts has not yet been explained: namely, hermaphroditism. By hermaphroditism is meant the existence of complete and separate sets of female and male gonads in the same individual. This condition exists regularly not only in definite groups of animals, _e. g._, certain snails, leeches, tape-worms, but also, as everybody knows, in flowering plants. While in some forms both kinds of sex cells, male and female, are formed and mature simultaneously, as, _e. g._, in the Ascidian _Ciona_ (see Chapter IV), in others they are formed successively, very often the spermatozoa appearing first (protandric hermaphroditism). In the long tapeworm _Tænia_ each ring has testes and ovaries, but the young rings are only male while in the older rings the testes disappear and the ovaries are formed. The same ring is in succession male and female. How can we reconcile the facts of hermaphroditism with the chromosome theory of sex determination? _Rhabdonema nigrovenosum_, a parasite living in the lungs of the frog, is hermaphroditic, but its eggs produce not a hermaphroditic generation but one with the two separate sexes; this generation is not parasitic and lives in the soil. The generation produced by these separate males and females gives rise again to a hermaphrodite which migrates into the lungs of the frogs. According to Boveri and Schleip[188] the cells of the hermaphrodite have twelve chromosomes. It produces two types of spermatozoa with six and five chromosomes respectively (one-half of the cells losing one chromosome which is left at the line of cleavage between the two cells); and one type with six chromosomes. In this way separate males and females are produced by the hermaphrodite, females with twelve and males with eleven chromosomes. [188] Boveri, Th., _Verhand. d. phys.-med. Gesellsch._ Würzburg, 1911, xli., 85. Schleip, W., _Ber. d. naturf. Gesellsch._, Freiburg i. Br., 1911, xix. The males produce again two kinds of spermatozoa, male and female producing, but the male-producing spermatozoa become functionless. This fusion of the other spermatozoön containing six chromosomes with an egg having six chromosomes leads again to the formation of the hermaphrodite with twelve chromosomes. It is obvious that in this case the cause for the hermaphroditism is not disclosed. If chromosomes have anything to do with hermaphroditism there must be an undiscovered element in the chromosomes which may explain why the female as well as the hermaphrodite have the same chromosome constitution; or we are forced to look for another determinant outside the X chromosomes or the chromosomes altogether. This seems to be the only cytological work on the problem of hermaphroditism. Experimental work has been begun by Correns[189] and by Shull on the determination of hermaphroditism in plants but lack of space forbids us to give details.