RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 45 



nected channel system, but is composed of longer and shorter 

 segments still independent of each other. 



These finally become confluent, to form the main systemic 

 lymphatic collecting trunks, and then only do these establish 

 their final junction with the jugular lymph sacs, through whose 

 interposition, as above stated, they gain in the typical mammal 

 their permanent entry into the venous system. 



In this ontogenesis of the systemic lymphatic vessels certain 

 relations between them and the venous system deserve further 

 notice. 



In the early stages the lymphatic mesenchymal spaces form a 

 wide meshed network (cf. series 111, figs. 1 to 7, series 120, figs. 

 8 and 9). There is thus a marked similarity in the earliest stages 

 of both the haemal and the lymphatic vessels, for the peripheral 

 venous embiyonic pathways are in their corresponding stages like- 

 wise still largely in the condition of a capillary reticulum. As the 

 main lines of venous drainage crystallize out of the antecedent 

 plexiform arrangement, the adjacent enlarging lymphatic channels 

 crowd in on the condensing venous line and continue the close re- 

 lationship which the earliest lymphatic anlages maintain to the 

 adjacent veins. Thus the main embryonic venous channels de- 

 velop along certain definite hydrostatic lines by enlargement and 

 confluence of the individual plexiform elements of the indefinite 

 antecedent network occupying these lines. The capillaries out- 

 side of these lines retrograde, so that the area of crosssection of 

 the defined venous channel is less than the cross-cut area of the 

 plexiform network which it replaces. 29 



The distinct impression is given that the space thus vacated 

 by the condensation of the plexiform venous network of the ear- 

 lier stages affords to the replacing lymphatic plexus the oppor- 

 tunity for greater growth and expansion, and that subsequently, 

 in repetition of the process previously active in the venous reticu- 

 lum, the lymphatic network condenses in a similar manner into 

 more defined channels along similar hydrostatic drainage lines, so 

 that the newly established main lymphatic vessel now closely 

 follows the main venous channel. It is to be noted, however, 

 that this organization of main vascular channels is usually less 



