510 



A 



B 



Fig. 1. Graphic reconstructions from rabbit embryos. A, lO'/., d^iys. 4,4 mm, 

 Harvard Embryological Collection, 559. B, 11 days, 5,0 mm, H. E. C, 556. C, 12 days, 

 5,0 mm, H. E. C, 104. D, 12'/., days, 7,0 mm, H. E. C, 149. D.Ao dorsal aorta. 

 V.Ao ventral aorta. IV fourth aortic arch. P.A pulmonary arch. P.IV fourth pha- 

 ryngeal pouch. P.B postbranchial body. 



