380 FRANK R. LILLIE 



It may, however, be objected that we are relying: on a rule which 

 has not been proved for cattle, viz: that in a single pregnancy 

 only one corpus luteum is present. To settle this question 

 examination was made of 81 uteri each containing a single calf; 

 in 45 of these both ovaries were present, and in every case only 

 a single corpus luteum was found; in 36 cases one ovary only 

 was present and 18 of these contained a single corpus luteum 

 each, the other 18 lacked a corpus luteum. There can, there- 

 fore, be no doubt that the rule holds for cattle. This conclusion 

 necessitates the inference that the monochorial condition of cattle 

 twins is secondary. In a later section we shall consider the 

 question of the probable time and nature of the fusion of the 

 two chorions. 



The conclusion drawn from the embryological evidence that the 

 heterosexual cattle twins are dizygotic is supported by two other 

 important considerations, viz : lack of close resemblance between 

 the bull and heifer of such pairs, and by the sex ratios of all cattle 

 twins. 



2. Concerning the degree of resemblance of such tunns 



A careful study of this point remains to be made; but it is 

 noteworthy that no one has recorded resemblances similar to 

 those of identical twins in the case of the free-martin and its 

 bull mate. My own somewhat limited observations lead me to 

 the conclusion that the resemblances are no closer than ordi- 

 nary fraternal likeness. One striking case was recently observed 

 by me in pure bred Holstein-Friesian cattle in w^hich the free- 

 martin was about half black and white in patches, and the male 

 was almost entirely black except its forehead and legs below the 

 knees. 



3. Sex-ratio of cattle twins 



On this point we have observations by Cole and myself. Cole 

 ('16) finds in a study of records of 303 multiple births in cattle 

 that there were 43 cases male twins, 165 cases two-sexed twins 

 (male and female), 88 cases female twins, and 7 cases of triplets. 

 This gives a ratio of about 1 d" d" : 4 cf 9 : 2 9 9 f or the twins, 

 instead of the expected ratio of 1:2:1. Cole then states : 



