32 THE BIOLOGY OF TWINS 



mechanism is adapted to accommodate a single develop- 

 ing egg. 



There is nothing suggestive of polyembryony about 

 the ovaries or oviducts. Each ovary, when no corpus 

 luteum is present, is kidney-shaped and about the 

 size of a small bean; in virgin females the two are of 

 the same size. In every pregnant female, however, 

 one of the ovaries is several times as large as the other, 

 owing to the presence of an enormous corpus luteum 

 in that ovary which has produced the fertilized ovum. 

 As in many mammals, the corpus luteum of pregnancy 

 is an extremely conspicuous object, especially in its 

 earlier phases, and its presence is unmistakable. One 

 of the earliest and most important discoveries in the 

 armadillo investigation was that there is never more 

 than one true corpus luteum in the ovaries of a pregnant 

 female. That the number of corpora lutea is a safe 

 criterion of the number of eggs involved in a pregnancy 

 is generally recognized by embryologists, and we may 

 feel safe in applying this test in cases such as those 

 offered by human and by ungulate twins where the 

 early embryonic history is unknown. The evidence 

 of the corpus luteum that, in the armadillo, only one 

 egg is produced and fertilized at a pregnancy is supported 

 by a study of ovogenesis, maturation, and fertilization. 



OVOGENESIS 



The process of ovogenesis^ (development of eggs) 

 in the armadillo presents in the earlier stages at least 

 nothing unusual, but is, on the contrary, quite typical 

 for mammals in general. Each of the young ovocytes 



^ H. H. Newman, Biological Bulletin, XXIII (191 2). 



