210 Biology in America 



result is the same, namely, an uneven distribution of chromo- 

 somes to different sex cells, indicating clearly a separation 

 of entire chromosomes from one another in maturation, and 

 the production of different kinds of sex cells as a result 

 thereof. Now when a sperm carrying a larger number of 

 chromosomes, or a larger member of an unequal pair, unites 

 with an egg, the resulting offspring is a female; while those 

 sperms which carry fewer or smaller chromosomes, upon 

 fertilizing an egg, give rise to males. 



In most cases studied thus far the differential divisions 

 occur in the male, the sperm being of two classes, male and 

 female producing; but in a few animal;, notably birds, mrtiis 

 and butterflies, the eggs are of two clas ;es and sex determina- 

 tion occurs in the female. 



While the greatest mass of evidence available thus far 

 indicates that sex is predetermined in the fertilized egg, there 

 is some recent work which does not apparently agree with 

 this theory. A consideration of this evidence however may 

 best be deferred to a later chapter. 



There are also two sexual conditions of more or less common 

 occurrence in plants and animals, which are difficult to explain 

 on the basis of sex chromosomes. One of these, hermaphro- 

 ditism, is of very general occurrence in plants and certain 

 groups of animals; and the other, gynandromorphism, occurs 

 occasionally in animals. In the former case both sex glands 

 are normally present in the same animal ; in the latter vary- 

 ing conditions of the glands occur, sometimes male, sometimes 

 female, sometimes both are present, while externally the body 

 may be of different sexes on opposite sides, or opposite ends, 

 or one-fourth may differ from the remaining three-fourths ; 

 in fact, almost any mosaic of external sex characters may 

 occur. 



Hermaphroditism is characteristic of most worms, and some 

 molluscs, and occurs occasionally elsewhere. In vertebrates 

 it is rare, being characteristic only of the hagfish (Myxine). 

 In mammals true hermaphroditism is not known, so-called 

 hermaphrodites having only the external sex organs hermaph- 

 roditic. Various hypotheses have been advanced to bring 

 these conditions into line with the chromosome hypothesis, but 

 thus far without any marked degree of success. 



By far the most valuable contribution of recent years to 

 the chromosome theory of inheritance is the work of Morgan 

 and his students at Columbia on the fruit fly, Drosophila. 

 By a combination of breeding and cytological studies they 

 have carried this theory almost to the point of fact. Droso- 

 phila is a little fly, about half the size of the ordinary house 

 fly, which breeds abundantly in decaying fruit and vegetables, 



