io8 THE BIOLOGY OF TWINS 



in those mammals that unquestionably show it, has a 

 definite relation to certain pecuHar types of simple uteri 

 and a special type of embryonic development called 

 germ-layer inversion. In the armadillo it is difficult to 

 imagine how, apart from germ-layer inversion, the 

 complete separation of embryos could occur and the 

 monochorial condition be still maintained. In man 

 the conditions described for the Bryce and Teacher 

 ovum appear to indicate a situation like that of the 

 armadillo; but no such condition has been described for 

 any of the ungulates. It is barely possible that blas- 

 tomeres could separate and form two individual embryos, 

 but this would involve a subsequent fusion of chorions just 

 like that which occurs in the dizygotic twins of Euphrac- 

 tus villosus. Lillie cited one equivocal case of apparent 

 monozygotic twins. In one pair of twin males the 

 genitalia of the mother were roughly handled so that 

 when examined but one ovary was present. This 

 ovary had no corpus luteum. The inference made is 

 that the lost ovary had but one corpus luteum, since 

 in all other cases examined but one corpus luteum 

 occurs to an ovary. It seems probable, however, that 

 the lost ovary would have shown two corpora lutea 

 had it been examined. That an ovary may ovulate 

 two ova at a time is seen from the fact that triplets, 

 which may be either of the same sex or of opposite sexes, 

 not infrequently occur. That calf twins may be 

 monozygotic is believed by several writers. Pearl, 

 for example, in a paper on '' Triplet Calves" states 

 that the two nearly identical females (see two outer 

 calves in Fig. 40) are possibly, if not probably, derived 

 from the first two blastomeres of a single dividing egg. 



