110 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



agus magnified 225 diameters. The prevertebral venous plexus 

 (17) extends in an arch concentric with the dorsal circumference 

 of the oesophagus (8), in front of the vertebral centers. Between 

 it and the oesophagus, mesal to the sympathetics (1), are seen 

 on each side former elements of the venous plexus (4) surrounded 

 by the extraintimal lymphatic anlages (5) of the preazygos seg- 

 ment of the thoracic duct. 



Figs. 164, 165 and 166 show the same structures in the next 

 three sections (25, 24 and 23), proceeding cephalad. Fig. 166 

 is especially clear. In it the original line of connection of the 

 atrophying venous kernel (4) with the prevertebral plexus (17) 

 can still be made out as a band of condensed mesoderm, and 

 the endothelium of the perivenous lymphatic space (5) is clearly 

 marked. 



Figs. 167 and 168 show this area in two successive sections of 

 the same embryo further caudad (series 210, slide ix, sections 

 43 and 44, X 150), and fig. 169 gives the last section magnified 

 200 diameters. The empty central venous remnant (4), with 

 the enveloping extraintimal lymphatic anlage (5), and the still 

 functional elements of the prevertebral plexus (17), can be seen 

 in all of the sections. They also show, nearer to the dorso- 

 lateral wall of the oesophagus, one of the collections of darkly 

 stained mesodermal cells above referred to as representing rem- 

 nants of formerly patent blood-vessels. 



The series of microphotographs shown in figs. 160 to 169 

 inclusive again conclusively demonstrates the origin of this 

 portion of the lymphatic channel by confluence of independent 

 extraintimal mesenchymal spaces surrounding and replacing 

 degenerating embryonic venules. The reproductions only give 

 relatively faint and indistinct views of the actual conditions ob- 

 served under the microscope. The general mesenchymal tissue 

 in this region is extremely loose and the area is very difficult 

 to photograph sharply and distinctly. The actual sections, 

 however, offer histological pictures identical with those observed 

 as above described in the development of the ventral mediastinal 

 lymphatic duct. The extraintimal lymphatic spaces are either 

 closely applied to part of the circumference of the wall of the 



