86 THE BIOLOGY OF TWINS 



A much more satisfactory explanation is associated 

 with the fact that there is an early "period of quies- 

 cence" of the egg. This fact, though no significance was 

 attributed to it, was brought out by Patterson, who 

 found that all of the eggs collected over a period extend- 

 ing from the middle of October to the fourth of Novem- 

 ber were in almost the same stage of development. It 

 was found, moreover, that eggs in the Fallopian tubes 

 were almost as advanced as those found practically in 

 the area of placentation. These observations prove 

 that the factors responsible for retardation of develop- 

 ment in the polyembryonic species are in operation at 

 a very early period, probably as early as the first cleavage 

 stages. The problem is to locate the factors respon- 

 sible for the slowing down of the developmental rhythm. 

 Whatever these factors may be, and we have no definite 

 knowledge of them, the result of retardation is poly- 

 embryony. 



For some years I have been convinced that this 

 case of agamic reproduction is physiologically equiva- 

 lent, in some important respects, to the well-known 

 case of budding in plants. This view was expressed 

 in 19 13 in a general paper^ on the natural history of the 

 nine-banded armadillo. In that paper I ventured, 

 perhaps unwisely, to attribute the retarded develop- 

 ment to the presence of a specific parasite in the arma- 

 dillo egg. This structure had at that time been diag- 

 nosed for me by an expert protozoologist as a sporozoan 

 parasite. At the present time I am unable to decide 

 whether the object in question is a protozoan or not; 

 at least it is of universal occurrence in the armadillo egg, 



^ H. H. Newman, American Naturalist, XLVII, 1913. 



