40 THE PHYSIOLOGY OF TWINNING 



double (and multiple) monstrosities, or for double (and multiple) 

 unioval separate embryos. 



These different modes are: (i) The appearance of two (or 

 more) embryonic rudiments on a single blastoderm. (2) The 

 presence in the egg of two (or more) separate blastoderms. (3) 

 Fission or dichotomy on the part of a single embryonic rudiment. 

 (4) Formation of certain axial structures in two parallel sets on a 

 single embryonic rudiment. 



Modes I and 2 are, doubtless exactly equivalent to 

 the similar modes described in chapter i for the starfish. 

 Modes 3 and 4 are, I believe, merely different degrees of 

 the same process of longitudinal fission, which involves 

 the more or less complete bilateral separation of the 

 two sides of the bilateral blastoderm. 



Gemmill considers that the first mode is universal 

 for fishes, that the second is represented, in fishes, only 

 by a single instance in the Hterature (Klausner's case 

 cited below), and that the third and fourth modes of 

 origin are extremely rare in fishes. 



There is Httle doubt, I beheve, that separate one-egg 

 twins may and do originate by means of both of the 

 first two modes. Several writers have described instances 

 of germ-ring stages in which there were two or more 

 embryonic shields. Rauber, especially, has given us 

 unequivocal examples of this mode of origin as shown 

 in Figures 23 and 24. Such examples as these are 

 evidently instances quite completely equivalent to the 

 commonest type of twinning in the starfish, involving a 

 loss of axiation of the single blastoderm and the produc- 

 tion of two or more regions of gastrulation instead of 

 the origmal one. Rauber's second figure (Fig. 24) shows 

 on the right side what I believe is an early stage of a 

 double monster resulting from the dichotomy of an origi- 



