484 MISSES K. FOOT AND E. C. STROBELL : 



14 chromosomes that is never present in the female, and is present in a/£ 

 the so-called male-producing spermatozoa, while each o£ the 13 autosomes- 

 (according to their accepted mode of division) can be present in half tlie 

 male-producing and half the female-producing spermatozoa, and therefore in 

 both half the males and half the females. 



Even the defenders of the chromosome sex-determination hypothesis reject 

 this Y chromosome as the carrier of factors essential for the determination 

 of sex, for the very cogent reason that in so many forms no Y chromosome 

 is present. Morgan ('11) concludes that " the factors for producing the- 

 male must be located in some other chromosome." 



As the Y chromosome is thus rejected as a sex-determiner, it would seem 

 that those who hold that factors essential for the development of definite- 

 characters are carried by definite chromosomes are forced to assign to the 

 Y chromosome (in forms in which it is present) the function of cari-yiug 

 fiictors essential for the development of characters exclusively male, since^. 

 as stated above, it is the only chromosme that is always present in the male- 

 and absent in the female. We have shown, however, that such exclusively 

 male characters as the genital spot and the intromitteut organ can be- 

 inherited without the Y chromosome, and this certainly challenges such an 

 interpretation, and leads us to respectfully ask the advocates of the chromo- 

 some-hypotheses what characters they would assign to it. 



An ingenious apology for the obvious shortcomings of the sex-chromosome 

 hypothesis has been recently made by C. B. Bridges, '13 (one of Morgan's- 

 pupils). It is offered as an explanation of occasional slips in linkage. 

 Bridges found in Drosopldla ampelopliila that two sex-linked characters 

 (red eye and white eye) failed to show linkage in 5 per cent, of the cases 

 (Wilson, '14, states that these exceptions are " about 10 per cent."). 



To the class of cytologists to which Bridges belongs, ''sex-linked 

 characters" are in reality X-Unked, and therefore, for example, it is' 

 theoretically impossible for a male to transmit directly to his male offspring 

 a character that is assumed to be carried by the X chromosome — the- 

 chromosome that is absent from the male-producing spermatozoon. lu' 

 order then to explain away these embarrassing slips in linkage, Bridges has- 

 subniitted the following ingenious explanation, which is based on the- 

 admission that the X chromsomes do not move during maturation with that 

 military precision heretofore demanded by the theories. He now suggests 

 each may show an equal amount of erratic movement — the one destined for- 

 the polar body may remain in the egg, and the one destined to remain in 

 the egg, may go with its mate to the polar body ; and thus three kinds of. 

 ripe eggs are possible —the first with the usual one X, the second with no X. 

 at all, and the third with two X's. This erratic behaviour of the X chromo- 

 somes can be made to account for many disappointments in expected results,. 



