TRIPLET CALVES. 261 



ing about the physiological causes which determine that in the 

 breeding history of a particular individual, twins are born at, 

 let us say, the fourth pregnancy, while from all other pregnan- 

 cies in the life of the animal only single young come. From 

 whatever angle we approach the matter it is clear that there 

 must be a definite (and presumably determinable) set of causes 

 in the physiology of either the ovaries or Fallopian tubes and 

 uterus, or both which leads to the production of twins. In the 

 hope of getting light on this problem the collection of the 

 most complete information possible in regard to multiple gesta- 

 tion in normally uniparous forms is to be desired. This is a 

 matter in which stock breeders may give valuable aid. 



2. The problem of sex determination is one very much in 

 the foreground of biological interest at this time. The work 

 of the last 10 3^ears seems to have made some really definite 

 progress towards the final solution of the problem. A number 

 of dififerent lines of research just now being prosecuted vigor- 

 ously all lead to the same general view as to the basis of the 

 causation of sex. Tn the first place the cytological researches 

 of McClung, Wilson, Stevens and Morgan,* and others indicate 

 that in many forms of life, at least, there are constant and char- 

 acteristic differences between the sexes in respect to the num- 

 ber, form and size of the chromosomes and that further the 

 behavior of the chromosomes of the reproductive cells during 

 their maturation and fertilization is of the sort which would be 

 expected to occur if, in the first place, sex were inherited and, 

 in the second place, this inheritance were controlled by certain 

 of the chromosomes. Supplementing this cytological evidence 

 and leading to the same conclusion that sex is a definitely in- 

 herited character, are the experimental researches in Avhich 

 sex is shown to behave in the same manner that structural char- 



■"It is not necessary to cite in detail here references to all papers of 

 the workers in this field. An extensive bibliography is given in Morgan, 

 T. H., "A Biological and Cytological Study of Sex Determination in 

 Phylloxerans and Aphids." Jour. Exper. Z06I. Vol. vii, pp. 239-352, 

 1909. Summary accounts of the cytological work on sex determinatioii 

 are to be found in Wilson, E. B. "Recent Researches on the Determi^ 

 nation and Heredity of Sex," Science, N. S. Vol. 29, p. 53, 1909 and 

 in Doncaster, L., "Recent Work on the Determination of Sex." Science 

 Progress, No. 13, pp. 90-104, July 1909. See also the more recent 

 summary by Professor Wilson, "The Sex Chromosomes," Arch. mik. 

 Anat. Bd. TJ, pp. 249-271, 191 1. 



