Earliest Blood Vessels in Anterior Limb Buds. 295 



considers the story told in the earliest stages (embryos 1 and 2), 

 it is evident that we -must consider the umbilical vein as its earliest 

 drainage channel. This vessel grows rapidly in size and receives 

 a constantly increasing number and caliber of venules from the 

 limb. But a process of coalescence of the limb's capillaries with 

 the posterior cardinal vein opens up other avenues of drainage, so 

 that there comes to be a row of venules opening from the capillary 

 plexus of the limb bud into the posterior cardinal vein. In embryo 

 3, there are four of these venules on the right side but many others 

 are soon to be formed. 







DorScOArt .-•" I 1 -i.1^ ^Ou .yyg\ 



v< 



X* 



Fig. 4.— Crogs section of chick embryo of 33 somites (embryo 7 of table), 

 showing the first left subclavian artery, X 531/4. 



Sp. Ch., spinal cord. 



Dor. Seg. Vein, dorsal segmental vein. 



Dor. Seg. Art., dorsal segmental artery. 



Subcl. Art., subclavian artery. One notes its origin from the mid-lateral 

 aortic wall. 



Nep. Art., nephric artery. 



The next embryo, No. 4, is remarkable in possessing the greatest 

 number of subclavian capillaries found in the series (Fig. 5). 



