ILLUSTRATIONS 



[Drawingrs by J. Castro.] 



Plate I 



Fig. 1. Dorsal view of duck twins, set III. pa, proamnion; ap, area pel- 

 lucida; mg, mg', medullary groove; np, anterior neuropore; ao, 

 area opaca; cr, cardiac region; cmg, caudal portion of medullary 

 groove still open. 

 2. Ventral view of duck twins, set III. ao, area opaca; fc, fovea 

 cardiaca; ompv, ompv', omphalomesaraic vein; mv, median vein 

 between the two embryos; som, somites. 



Plate II 



Dorsal view of duck twins, set II. ao, area opaca; pa, proamnion; hp, 

 hp', head process; ap, area pellucida; som, som', somites; mg, 

 medullary groove; cmg, medullary groove in common. 



Plate III 



Fig. 1. Dorsal view of duck twins, set I. ao, area opaca; ap, area 

 pellucida; hp, hp', head process; nc, nc', notochord; fi, line of 

 fusion between the two embryos. 

 2. Dorsal view of duck twins, set IV. ap, area pellucida; ao, area 

 opaca; hp, head process; If, line of fusion between two embryos; 

 hpd, head process disintegrated; mg, medullary groove; som, 

 somites. 



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