426 OTTO F. KAMPMEIER 



Those that were not hidden in the dorsal view of the reconstruc- 

 tion are illustrated in the drawing (4-, fig. 28). These spaces are 

 short capsule-shaped vesicles running through four or five sec- 

 tions and are quite distinct in outline, so that their extent and 

 discontinuity can be easily determined. A transverse section 

 of one of them is represented in figure 7 (4). The nicety with 

 which it can be discriminated from the interstices of the sur- 

 rounding mesenchyme and from the lumen of the neighboring 

 venule which is filled with blood removes all doubt as to its reality 

 and individuality. 



On the evidence of the few microphotographs inserted here, 

 the presence of the spaces pointed out as lymphatic anlagen 

 can not be denied. In the embryo just described they exist only 

 in the foremost region of the thoracic duct line; outside of this 

 path there are no spaces which might invalidate the significance 

 of these anlagen. 



Extra-intimal replacement of evanescent venous channels, a 

 method of lymphatic development just hinted at in figure 5 (4) 

 finds a most convincing expression in series 194 (20 mm.), where 

 the entire anterior precardinal veno-lymphatic plexus (6a) is 

 being replaced by large perivenous spaces (3, fig. 29). The 

 veno-lymphatics designated have been detached from their 

 parent veins and thus abandoned by the systemic blood vascular 

 circulation, and they now display successive steps towards com- 

 plete collapse. The section represented in figure 8 is typical, for 

 most of the other sections taken at random from this region offer 

 equally decisive illustrations. Besides revealing the shriveled 

 and discarded venous intima and its gradual disintegration in the 



Fig. 6 Transverse section through the left lower cervical region in a 19 mm. pig 

 embryo (series 168, slide 18, section 4). X 150. 4, vacuolation of the mesenchyme 

 in the formation of lymphatic spaces; 6a, precardinal veno-lymphatics, beginning 

 to degenerate; 9, internal jugular; 13, aorta; 14, sympathetic nerve and branches; 

 17, oesophagus. (Reconstruction, fig. 28.) 



Fig. 7 Transverse section through the left upper thoracic region in a 19 mm. 

 pig embryo (series 168, slide 19 section 1), X 150. 4, lymphatic space in the 

 line of the future right thoracic duct; 7, oblique vessel; 8, dorsal segmental vein; 

 10, left precardinal; 13, aorta; 14, sympathetic nerve trunk and branch; 17, oeso- 

 phagus. (Reconstruction, fig. 28.) 



