PLATE S 



EXPLAXATIOX OF FIGURE 



Ul From a section of a chick embi\vo of 6 days and 22 hours, 16 mm. (series 

 no. 463; sliiU' X\'I, section 9). Photomicrograph, X 160. 



£, dorsal aortic root 15, jugular lymph sac 



3(1. pulmonary artery 15a, thoracic duct 'approach' 



12. precaval vein 27, vagus nerve 



13, vertebral vein 



178 



