100 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



ing only the clear lymphatic lumen (5). In other places remnants 

 of the replaced vein (4) can still be distinctly determined. This 

 clearance has resulted in a relative diminution of the actual area 

 of a cross section of the meshes of the lymphatic plexus, since now 

 the entire lumen is given over to the newly formed lymphatic 

 channel, whereas in the earlier stages its central area was still 

 largely filled by the degenerating veil. The lymphatic spaces 

 in these sections have increased considerably in number. (Only 

 a few of the more prominent vessels are indicated in the figures 

 by leaders.) 



A transverse section of this region in a 15.5 mm. cat embryo 

 (series 215, slide xiv, section 31, X 225) is shown in a fig.146. 

 The preceding more diffuse lymphatic plexus of the earlier stages 

 is condensed to a much smaller mesenchymal network. In a 

 few of the larger meshes a remnant of the former central vein (4) 

 can still be traced as a solid plug of mesoderm surrounded by the 

 lymphatic channel (5). 



In general the 15 and 15.5 mm. embryos of the cat represent 

 the stages in the development of the broncho-mediastinal trunk 

 in which the histological pictures of the lymph channels are least 

 distinct and striking. This is due to the fact that on the one 

 hand they have lost the central venous kernel around which they 

 were originally developed through confluence of extraintrmal 

 spaces, and which gave to the combined structures their area 

 on section and their distinctive appearance in the earlier (13.5 

 and 14 mm.) stages, while on the other hand the independent 

 growth and enlargement of the lymph channels proper begins in 

 the 16 mm. stage. In some embryos, measuring 16 mm., the 

 appearances characteristic of the 15 mm. and 15.5 mm. stages are 

 still encountered. Thus fig. 147 shows a transverse section 

 through the ventral area of the upper thoracic region in a 16 mm. 

 embryo (series 230, slide xii, section 25, X 225). The corre- 

 spondence of the histological picture to that presented by the 

 15 mm. or 15.5 mm. embryo is seen by comparison with series 

 215 and 216 (figs. 142 to 146). 



The majority of 16 mm. embryos show, however, a marked 

 advance in the independent growth of the lymphatic channels. 



