262 F. M. JEWELL. 



they corresponded in number to the number of foetuses with one 

 exception. This exception was a pair of identical twins from 

 one ovum, and since we are concerned with the sex ratio at the 

 time of conception, obviously only one of these should be recorded 

 in the data. In this case both ovaries were present, there was 

 only one corpus luteum,.and the twins of course were in one 

 horn of the uterus and both of the same sex. This case of 

 monozygotic twins is of very rare occurrence in cattle, being the 

 first observed in the collection of 108 twins in the Zoology 

 Department of the University of Chicago. There were four 

 pairs of twins in the 1,000 foetuses, and as is commonly the case 

 in twins of opposite sex, the male is usually a little farther along 

 in development than the female. In numbers 646 and 647 the 

 male was 3.5 cm. longer than the female, the latter being 65.0 

 cm. long. In 807 and 808 the males were both the same length, 

 68.5 cm. In 988 and 989 the male was 6.1 cm. longer than the 

 female, the latter being 58.2 cm. long. 



Twins Numbers 648 and 647 and also 807 and 808 were in 

 separate horns of the uterus and there was one corpus luteum 

 in each ovary; while numbers 988 and 989 were in one horn of 

 the uterus and there were two corpora lutea in the ovary of 

 that side. Since the ovaries on some days were removed for 

 commercial purposes before I had access to them, it was impos- 

 sible to obtain data of this kind on the total of 1,000 fcetuses. 



C. Analysis of Data. Since the length of the embryo can be 

 used as an indication of its age, we can arbitrarily make certain 

 groupings and assume that those within that group are on the 

 average at about the same age. 



In Table I. is given such a grouping according to length, from 

 O-IO cm., 10-20 cm. etc., up to 90-100 cm. The smallest in- 

 dividual was 4.2 cm. in length and the largest was 95.3 cm. in 

 length. Thus the data extends from a comparatively early 

 period in fcetal development practically to the time of birth. 



The lengths of embryos were tabulated in lots of 50 in order 

 to get some idea of how the sex ratio would vary according to 

 the position of the group in the total of 1,000 fcetuses. Thus 

 in the first space in Table I. are given the inividuals from no. i 



