PROBLEMS OF MAMM.\LIAN DEVELOPMENT 440 



such a scheme in Figs. 113 and 114 will give a mechanism of sex 

 determination which might be called the exception that proves 

 the rule. In a more exceptional type which is found in some 

 moths, and perhaps in birds, the female possesses the single sex 

 chromosome and the male the paired condition, so that there are 

 two kinds of ova, one with and the other without an X-chromosome, 

 while all spermatozoa have this chromosome. 



As the investigations have been continued, they have revealed 

 many lesser exceptions to the general scheme, which complicate, 

 but do not overthrow, the fundamental relationship between sex 

 and the number of chromosomes. A more important modifica- 

 tion of the original concept is necessitated by the recent evidence 

 that sex-determining factors are also contained in some of the auto- 

 somes. It has also been shown, in some rare instances, that the 

 sex of an individual may be reversed, as in poultrj^, where an 

 apparently functional female is said to have become a male, and 

 in some of the fishes and Amphibia where the sex may be nonnally 

 reversed as the individual grows older. In a few instances it has 

 even been shown that the sex may be reversed experimentally by 

 changes in environmental conditions. These complications of the 

 fundamental theory cannot be detailed in so brief a discussion. In 

 a modified form, the chromosome theory of sex detennination 

 now seems well established, although it is not so simple as was once 

 supposed or as might seem from the examples that have been 

 chosen to illustrate the principle. According to this theory, the 

 maleness or femaleness of an adult animal is detennined primarily 

 by hereditary factors transmitted in the germ cells and not by 

 environmental influences during development. The discovery of 

 this relationship, and the consequent overthrow of the earlier 

 theory that the food of the embryo was the fundamental cause of 

 sex differentiation, further illustrates the difficulty of disentangling 

 hereditary and environmental influences, as explained in a pre- 

 ceding section. The hereditary factor is often difficult to recog- 

 nize, but, once discovered, it may be shown to exercise the greater 

 influence. 



Problems of Mammalian Development. — Prenatal Influences. 

 — The retention of the mammalian embryo within the body of 

 the female parent (Figs. 234 and 235), during the stages of develop- 

 ment corresponding to those of a bird or a reptile within the egg 

 (ff. Fig. 220, p. 423, and Fig. 224, p. 428), raises questions regard- 



