PLATE 1 



EXPLANATION OF FIG THE 



28 Reconstruction of the vascular channels of the lower cervical and thoracic 

 regions in a 19 mm. pig embryo (series 168, slides 16-24 inclusive), X 50. Dorsal 

 view. Arrows indicate the levels at which the microphotographs were taken. 

 Cross lines, not labelled, indicate the extent of the divisions, A, B, and C. 



A, precardinal division 



B, postcardinal division 



C, supracardinal division 



1, left jugular lymph sac 



2, thoracic duct approach 



4, lymphatic spaces, incipient thoracic 



duct anlagen 

 6a, precardinal veno-lymphatics 

 6b, postcardinal veno-lymphatics 

 6c, supracardinal veno-lymphatics 



7, oblique vessel 



8, dorsal segmental veins of the pre-, 

 post-, and supracardinals, respec- 

 tively 



9, internal jugular vein 



10, left precardinal vein 



lid, lis, right and left postcardinal 



veins 

 12d, 12s, right and left supracardinal 



veins 

 12ld, right supracardinal line beginning 



to fuse with the right postcardinal 



12ls, left supracardinal line, plexiform 



13, aorta 



13a, aortic arch 



13c, left carotid artery 



13ds, dorsal segmental arteries 



13s, left subclavian artery and branches 



14, left sympathetic nerve trunk 



15, vagus 



16, recurrent laryngeal nerve, and ac- 

 companying vein 



17, oesophagus 



20, mesonephroi 



21, left Cuvierian duct 



22, left subclavian vein 



23, cephalic vein 



24, external jugular vein 



25, venous plexus between supracar- 

 dinal lines 



26, subclavian approach of jugular 

 lymph sac 



N5, fifth spinal nerve 



466 



