No. 524] CHROMOSOMES AND HEREDITY 491 



problem of the four X's; or we may assume that despite 

 their similarity in size they are nevertheless qualitatively 

 different. In this. case we should still speak of one of 

 them as Y, and imagine that when a sperm bearing a Y 

 enters an egg a male results. In favor of the latter inter- 

 pretation Wilson has pointed out that an unbroken series 

 of forms exists at one end of which the X-chromosome 

 has no partner, in the middle of the series a partner of 

 unequal size, toward the other end of the series a partner 

 of nearly equal size, and at the end of the series a partner 

 of equal size. If we are justified in attributing the male 

 sex to no X, or to Y, it may seem that when Y can no 

 longer be distinguished by its size it may still be respon- 

 sible for the production of maleness. On the other hand, 

 if X and Y do not in themselves produce sex, but simply 

 accompany more profound changes, they are only indices 

 of what is taking place and the graded series has no im- 

 portant significance. 



These cases all apply to the group of insects. The 

 criticism has been made that we are not justified in ex- 

 tending these conclusions to other groups where no such 

 difference in number of chromosomes exist. 



Quite recently surprising results have been obtained 

 in groups other than the insects, that go far toward meet- 

 ing the criticism just referred to. 



First Baltzer has found in the sea urchins that there 

 are specific chromosomes found only in the female. The 

 spermatozoa are all alike, but the eggs are of two classes. 

 In principle the outcome is the same except in so far as it 

 shows that the sex element may be confined either to the 

 male or to the female. Second. Guyer has found in the 

 fowl that there is an odd chromosome in the male. This 

 is the first case reported for the vertebrates, but the 

 chromosomes in the group are so numerous or the cells 

 so small that failure to detect two classes of sperm in this 

 group (if they exist) is not surprising. Lastly, the all- 

 important outstanding case of Ascaris has been brought 

 into line by the announcement within the last few weeks 

 by Boveri of the discovery of an accessory in this group. 



