ASYMMF.TRTf'AL BUPLIOITY IN THE flHICK. 99 



the (levelopmeut in proximity of two embryos, each derived from 

 a fertilised ovum. In this type secondary fusion mayor may not 

 occur. The latter signifies the development of two more or less 

 complete individuals from a single fertilised ovum, and involves, of 

 course, a process of fission or separation of the original germ into 

 two more or less autonomous centres of embryonal formation. 

 Either type may be specific, as in the armadillo, or merely 

 abnormal and sporadic, as in the chick and man. Both types 

 have been experimentally induced in lower forms, the former in 

 particular by Driesch and Metschnikoft" (3, 9), who have suc- 

 ceeded in producing a single larva from the fusion of two eggs 

 or young larvte, the latter by numerous investigatt)rs. 



As to which mode of twinning is responsible for the production 

 of the diembryonal blastoderms which are of so common occur- 

 rence in the chick, there would seem to be no general agreement, 

 both seem possible a j^riori, and the conclusions of the majority of 

 authors hitherfco, seem to be mainly surmise. Advocates of the 

 former theory in one or other of its numerous forms have been 

 the Hertwigs, Weidman and Tur, Broman, Windle, Panum, 

 Klaussner and Marchand, and of the latter numerous authors 

 including in recent times, Bateson, Kaestner, Wilder, &c. 



The present specimen seems of extreme interest as leaving no 

 room for doubt between these two alternatives. 



The following features of the blastoderm, considered in con- 

 junction, seem to indicate such a close morphogenetic relation- 

 ship between the two embryonal rudiments as can only be 

 construed as due to their having taken origin from an originally 

 single germinal centi-e, presumably by some process of fission or 

 division which resi\lted in the physiological autonomy of the 

 two secondary centres. 



1. The regular outline of the blastoderm as a whole, which shows 



no sign of division into two portions. 



2. The continuity of the germ-layers in the various regions of 



the blastoderm, and the continuity of the lateral coelomic 

 spaces in the region of Rudiment j3 with those in the region 

 of Rudiment a. 



3. The development of the two embryonal rudiments on a single 



longitudinal axis. The axis of the "primitive groove" of 

 Rudiment /? is situated on the anterior continuation of tho 

 median axis of Rudiment a (see PI. I. fig. 1). 



Further, along that portion of the longitudinal axis be- 

 tween the two rudiments the median band of unsplit meso- 

 derm forms what may be regarded in a sense as a differentiated 

 connection between the two rudiments, forming a conclusive 

 indication that they are indeed developed on what is strictly 

 a single primary morphogenetic axis. In other words, the 

 two embryonal rudiments must have arisen from the original 

 centre subsequently to the laying down or, at all events, to 

 the determination of the primary longitudinal axis. 



7* 



