SUPPLEMENTARY NOTES ON TWINS IN CATTLE. 5 1 



which, however, show a sex-ratio of only 111.5 counted as indi- 

 viduals. The approximation of the twin sex-ratio to the expected 

 sex-ratio is, therefore, sufficiently exact when the smallness of 

 numbers is taken into account to exclude the need of any special 

 assumption. 



Incidentally it may be noted what a complete demonstration the 

 figures furnish of the underlying assumption that the free-martin 

 is zygotically female, which D. Berry Hart apparently continues to 

 question (1918). In a recent paper Gowen (1921) presents ex- 

 tensive data showing that twins of cattle, whether of the same sex 

 or both, do not resemble one another in color more than full sisters 

 of the same breed. As the latter are, of course, dizygotic, the 

 presumption is that the twins are so likewise, or at least that 

 monozygotic twins in cattle are extremely rare. 



3. On Monozygotic Twinning in Cattle. 



The embryological criterion for distinguishing dizygotic and 

 monozygotic twinning in cattle would be the occurrence of two 

 corpora lutea in the maternal . ovaries in the former case and of ' 

 one in the latter. Implantation of the foetuses in one horn of the 

 uterus would also be expected, but this can not be relied upon by 

 itself as evidence, for one ovary often furnishes two ova, both 

 usually remaining in the horn of the uterus on the same side as 

 the ovary. It is also conceivable that two ova derived one from 

 each ovary might lodge in one horn of the uterus owing to passage ~ 

 of one of them from one horn to the other ; however, I have found 

 no such case. The existence of a single chorion would also, of 

 course, be postulated in monozygotic twinning, but its value as 

 evidence would be negligible owing to the tendency for originally 

 separate chorions to fuse in cattle. 



In the case of cattle, therefore, the embryological evidence for 

 monozygotic twinning is reduced to the occurrence of only one 

 corpus luteum for two embryos of the same sex. One such case 

 (No. 80) occurs in my collection. The chorion is single, re- 

 sembling in all respects cases in which the usual two corpora lutea 

 are from one ovary; the amniotic cavities were adjacent, but not 

 communicating; the twins were females of about 16 cm. greatest 

 length. The preparation has been preserved complete in formalin 



