94 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



arteries (10). The branches of the peritracheal venous plexus 

 (18) are well developed. There is as yet no indication of lym- 

 phatic development. 



2. In 13 mm. embryos certain of the venous radicles entering 

 into this plexus, are partly surrounded and enveloped by ex- 

 traintimal lymphatic spaces, which arise independently of the 

 venous system, as the first anlages of the future ventral medias- 

 tinal lymphatic channel. Figures 105 and 106 show transverse 

 sections of this region in a 13 mm. cat embryo (series 92, slide 

 vii, sections 30 and 33) in a magnification of 225 diameters. 

 Ventral to trachea (9), aorta (7) and left vagus (22) are seen sec- 

 tions of a number of venous radicles (4) of the ventral mediastinal 

 plexus, some of which are partially surrounded by very small 

 extraintimal perivenous lymphatic anlages (5). Only a few of the 

 most prominent of these are indicated in the figures by leaders, 

 but they exist in many other parts of the field. 



A similar section from another 13 mm. embryo (series 107, 

 slide ix, section 40, X 225) is shown in figure 107. Dorsal to 

 the pulmonary arteries (10), between them and the trachea (9) 

 and aorta (7), the section has cut a prominent extraintimal lym- 

 phatic space (5) in typical relation to one of the radicles (4) of 

 the ventral mediastinal venous network. 



3. In the 13.5 mm. embryo the full and convincing proof of 

 the extraintimal derivation of this channel is given. In some 

 embryos of the period the process is still seen in its earlier stages. 

 Thus figs. 108 and 109 (13.5 mm. embryo, series 223, slide x, 

 sections 11 and 12, X 225) show on each side, ventral to the 

 trachea (9) and the vagi (22), a number of radicles (4) of the ven- 

 tral mediastinal capillary plexus. One of these small vessels, 

 symmetrically disposed on each side in front of the respective 

 vagus, is in process of being surrounded and replaced by an ex- 

 traintimal lymphatic space (5). 



Other 13.5 mm. embryos show a rapid advance in the peri- 

 venous lymphatic development. 



Fig. 110 shows a transverse section of the upper thoracic 

 region of another 13.5 mm. embryo (series 189, slide viii, section 

 35, X 225). Just ventro-mesad of the left vagus nerve and its 



