Earliest Blood Vessels in Anterior Limb Buds. 



509 



an embryo of thirty-eight somites, belonging to the period of seg- 

 mental subclavians. Both embryos illustrate strikingly the facts 

 observed in these stages for chicks. 



Duck Embryos. 



The duck embryo of thirty-three somites was drawn carefuUly 

 from several aspects before being cut into serial sections. A gen- 

 eral view of the region of the anterior limb is shown in Fig. 14. 

 One sees clearly the irregular venous channel through the remains 

 of the primary body-wall plexus, leading now from the capillaries 

 of the limb to the duct of Cuvier. The umbilical vein in the duck 

 has an exactly similar origin as in the chick, and is here also the 

 primary drainage channel for the limb. Between the sixteenth and 

 twentieth dorsal segmental vessels an outgrowth of limb capillaries 

 from the aortic wall occurs. There are six of these vessels. They 

 anastomose promptly and form a continuous irregular capillary 

 plexus extending into the limli, which is as yet a mere swelling of 

 the somatopleure. ]^o one capillary of the subclavian series is 

 larger than its neighbor. The series is in no way arranged in a 

 segmental plan. 



Cross sections through the embryo show the topography of the 

 limb region. I have drawn one which shows strikingly the hith- 

 erto undescribed origin of these earliest subclavian capillaries from 

 the mid-lateral region of the aortic wall. They are compelled to 

 bend dorsally in growing into the limb bud. The section (Fig, 

 15) shows the fourth left and the sixth right subclavians. Xeither 

 vessel happens to arise at a segmental point. 



Fig. 14. — Upper body wall of duck embryo of 32 soimites. Lettering as in 

 previous figures. Tbe wall of the aorta is concealed behind the vein. 



