560 J. T. PATTERSON 



INTRODUCTION 



The development of more than one individual from a single egg 

 while not a rare phenomenon among animals, is nevertheless of 

 much biological interest. It has become customary to classify 

 such types of development as asexual or agamic reproduction; but 

 obviously this term has come to include a variety of developmental 

 phenomena, which are exhibited in animals ranging from the 

 Protozoa to the mammals. Among the more common types of 

 agamic reproduction are ordinary binary fission, budding, cyclical 

 parthenogenesis, paedogenesis, and polyembryony. 



Upon the basis of certain evidence which has been brought for- 

 ward from a study of comparatively late embryonic stages, it has 

 been correctly concluded that the type of agamogenesis which has 

 become habitual in the Texas armadillo is that of polyembryony; 

 but so far no one has succeeded in demonstrating the validity of 

 this conclusion. The writer has in his possession a series of young 

 stages which covers the period of early embryonic differentiation, 

 and which represents the material upon which this paper is based. 

 An outline of the more general features of the work has already 

 been given in a preliminary paper (Patterson '12). The facts to 

 be presented in detail are not without a certain interest and 

 significance, not only because they raise to the dignity of an ob- 

 served fact the claim for polyembryonic development in the arma- 

 dillo, but also for the reason that they throw a great deal of light 

 upon related phenomena in other mammals. It is unusual to find 

 agamic reproduction in the highest class of animals, and a detailed 

 study of the history of this process is greatly to be desired. Fur- 

 thermore, there has been considerable speculation as to how 'iden- 

 tical twins' and similar types of development have arisen, and I 

 believe that these studies on the development of the armadillo 

 will at least indicate how these phenomena may have come about. 2 



2 It is a pleasure to acknowledge here my indebtedness to my friend Mr. F. L. 

 Whitney of the School of Geology for his able assistance in connection with the 

 photographic work. I am also grateful to Mr. F. Pfeiffer, who has greatly facili- 

 tated this work by his many successful efforts in obtaining material. 



