ILLUSTRATIONS 



[Drawings by J. Castro.] 



i 



Plate I 



Fig. 1. Ventral view of duck twins V. pa, proamnion; fc, fovea cardiaca; 

 Is, lateral somite; ca", cd, chorda dorsalis; ps, primitive streak. 



2. Dorsal view of duck twins V. pa, proamnion; an, anterior neuro- 



pore; mg', mg, medullary groove; Is, lateral somite; mv, median 

 veins; ap, area pellucida. 



Plate II 



Fig. 1. Dorsal view of pharynx and aortae from duck twins III. ph', ph, 

 pharynx; da', da, dorsal aortae; omv', omv, omphalomesaraic vein; 

 mv, median vein; veh, venous end of heart. 

 2. Ventral view of pharynx, tubular hearts, and aortas from duck 

 twins III. ph', ph, pharynx; th, tubular heart; ve, ventral end 

 of heart; va, ventral aorta; omv, omphalomesaraic veins; mv, 

 median vein. 



3. Ventral view of pharynx and aortae, duck twins V. omv', omv, 



omphalomesaraic vein; ph, pharynx; th', th, tubular heart; of, 

 opening of foregut; mv, median vein. 



4. Dorsal view of pharynx and aortae, duck twins V. Ida', Ida, lateral 



dorsal aortae; ph, pharynx; mda, median dorsal aortas; mv, median 

 vein. 



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