RELATION OF LYMPHATIC TO BLOOD- VASCULAR SYSTEM 25 



of the mesonephros and the subcardinal cross anastomosis, and 

 gives a topographical view of the region in which the first meso- 

 dermal intercellular lymphatic tissue spaces {77) appear, in the in- 

 terval between aorta {73), postcardinal vein {68) and ccelom (75). 



Fig. 2 is the same section of this embryo, magnified 300 dia- 

 meters, and shows the area to the left of the aorta. Between 

 this vessel {73) and the well defined left postcardinal vein (68) 

 dorsad, and the coelom cavity {78) ventrad, appear a number 

 of clear mesenchymal spaces {77) which do not communicate \\ith 

 the adjacent venous channels. Some of these spaces extend 

 from the ccelom angle dorsad along the lateral aspect of the 

 postcardinal. They are the first distinctly recognizable anlages 

 of the lymphatic system, and they develop, from their first 

 inception, as independent mesenchymal spaces, closely related 

 to the adjacent hsemal channels, but genetically independent 

 of the latter. 



Fig. 3 shows the same field in a magnification of 600 diameters. 

 The spaces are clearly cut, separate and distinct, and the limit- 

 ing cells are beginning to assume endothelial character. 



The relation between the hsemal channels and the develop- 

 ing adjacent lymphatic spaces can be clearly traced in succes- 

 sive sections of this same embryo proceeding caudad. 



Figs. 4, 5, 6, and 7 show, respectively, sections 7, 8, 9 and 10 

 of slide viii of series 111, magnified 300 diameters. 



In all of these sections the uniformity, and the distinct struc- 

 tural character of the primary Ijmiphatic tissue-spaces is clearly 

 visible in the same situation and in identical relation to surround- 

 ing structures. 



For comparison with the preceding series the same region is 

 shown in another 10 mm. embryo (series 120, slide ix, sections 

 25 and 26) in two successive sections, magnified 300 diameters, 

 in figs. 8 and 9. 



In these sections the same independent mesodermal spaces 

 {77) are seen in their typical relation to aorta (75), post-cardi- 

 nal vein {68) and coelom cleft {78). 



The interesting question of the relationship between these 

 early mesenchymal spaces and the coelom cavity can only be 



