452 OTTO F. KAMPMEIER 



early lymphatic stage (23 mm. embryo) were injected and there- 

 fore are favorable for a comparison of their reactions to the injec- 

 tion mass. The veins (lid, lis, 25, fig. 24), possessing perfect 

 endothelial walls, did not admit of any extravasations or even 

 of blurred outlines. In the case of the thoracic duct (5d), on the 

 contrary, the injection mass (Ex) passed freely from the lumen 

 into the surrounding tissue reticulum, as pictured clearly in figure 

 24, showing the absence of any definite wall at this early embryonic 

 period. 



Figure 25 is from the lower postcardinal division in a 26 mm. 

 embryo and reveals essentially the same features as portrayed in 

 figure 22 but shows even better perhaps the multilocular character 

 of the thoracic duct anlage. The formation of spaces from the 

 mesenchyme and their addition to the anlage is very clearly 

 expressed on the right side in the wedge-shaped territory (5d) 

 between the oesophagus (17) and aorta (13), and also on the left 

 side (5s) immediately ventral to the left postcardinal vein (lis). 



A later stage (23 mm.) in the development of the cisterna 

 chyli (5) is shown in figure 26. The vestiges of the antecedent 

 veno-lymphatics have completely disappeared, and only occa- 

 sional trabeculae indicate the originally extensive tissue parti- 

 tions between the early rudiments of the cistema. What is of^ 

 greater significance, however, is the ragged outline of the cavity, 

 the absence of any specialized endothelium, and the addition of 

 small mesenchymal spaces (4-) to its lumen, upholding therefore 

 in every respect the writer's contention that the cisterna chyli, 

 concordant with the anterior divisions of the thoracic duct, is 

 primarily and fundamentally a product of mesenchymal differ- 

 entiation. 



Fig. 22 Transverse section through the upper thoracic region in a 21.5 mm. 

 pig embryo (series 192, slide 21, section 11), X 200. 5d, right thoracic duct anlage 

 showing its enlargement by the addition of adjacent mesenchymal spaces; lid, 

 right postcardinal; 13, wall of the aorta; 17, oesophagus; (compare with fig. 23). 



Fig. 23 An accurate camera lucida drawing of a highly magnified area repre- 

 sented in fig. 22, X 266 (reduced from X 400). 5d, right thoracic duct anlage 

 showing its concentric growth from enlarged tissue spaces and the absence of a 

 specialized intima: lid, right postcardinal vein filled with blood and possessing 

 a well-defined endothelial lining; 13, wall of the aorta; 17, oesophagus. 



