ONE-EGG TWINS IN FISHES 45 



originate from separate embryonic axes or centers of 

 gastrulation and those that originate from the fission 

 of a single embryonic axis. As will be shown below, all 

 true anadidymi must be viewed as incomplete fission 

 products of a single axis. If this view be true, some 

 separate twins may well belong to the same series as 

 the anadidymi, but not in the sense of Gemmill, who 

 believes that all arise as separate centers of gastrulation 

 and that conjoined twins are products of fusion. 



ORIGIN OF AUTOSITE AND PARASITE TWINS 



It is just here that we may profitably turn back to 

 the conditions described for the starfish. It will be 

 remembered that very frequently the original axis of the 

 blastula is only partially obliterated so that the main 

 basal area, or region of gastrulation, persists while one or 

 more minor basal areas may have been established. Such 

 minor areas give rise to secondary archentera, smaller in 

 size and situated at regions where archentera would not 

 be expected, sometimes appearing at the opposite pole or 

 apical end of the blastula. In the starfish these second- 

 ary archentera persist for a time but, in most cases, are 

 completely inhibited and subsequently become pinched 

 off and absorbed. In a few cases the secondary archen- 

 teron may be nearly as large and active as the primary, 

 and may persist as long as the larva lives. Now in the 

 fishes something very much like this probably takes 

 place. When the inhibition has been insufficient entirely 

 to obliterate the original axiate relations of the blasto- 

 derm, the original gastrulation area persists; but a 

 secondary area arises probably at the opposite side of 

 the blastoderm. This secondary area, after beginning 



