ORIGIN OF THE POSTERIOR LYMPH HEART 429 



of its vessels, prominent knob-like enlargements occurring where- 

 ever a large independent space previously existed. This plexus, 

 as has been noted, usually fills with stagnant blood, backed up 

 from the venous circulation. There is no connection between 

 the lyraiphatic plexus and the haemal circulation except at the 

 point where the lateral branches of the coccygeal veins have 

 just pierced the muscle plate. 



We now see that the lateral branches of the five or six most 

 cranial coccygeal veins pierce the muscle plate, drain the lym- 

 phatic plexus and then pass outward to drain the haemal capil- 

 lary plexus (fig. 13, 9a). Soon that portion of the lateral branches 

 of the coccygeal veins distal to the I3 mphatic taps degenerates, 

 thus severing the connection of these veins with the haemal 

 capillaries, so that those lateral coccygeal branches which drain 

 the lymphatic plexus, cease to function otherwise than for the 

 lymphatic drainage (fig. 13, -96). An examination of several 

 injected embryos cleared by the method of Spateholz showed 

 this point clearly; the haemal capillary plexus being drained in 

 the 15 mm. embryo by the most dorsal portions of the coccygeal 

 veins with only two of the lateral coccygeal branches assisting 

 them, although in the embryo of 11.5 mm. five or six lateral 

 coccygeal branches drained the plexus of haemal capillaries. 

 One 17.5 mm. embryo which was examined in cross sections, 

 showed no connection between the lateral branches of the five 

 or six coccygeal veins which drain the lymphatic plexus and the 

 haemal capillaries. 



Chick of 15 mm. Reconstruction of the caudal vessels. X 150. 

 Antero-lateral view. Figure 14- Arteries black, veins and capil- 

 laries white, lymphatics connected ivith the veijis green, isolated 

 spaces yellow. In this reconstruction the coccygeal veins (3) are 

 seen extending downward from the postcardinals {18) and the 

 coccygeal arteries (4) from the aorta {11). The coccygeal 

 veins give off lateral branches {9) which pierce the caudal muscle 

 plate — which has been omitted from this reconstruction — and 

 then proceed laterally to drain the lymphatic plexus (green) 

 and at the points where the lymphatics are not as yet formed 

 to any extent, the haemal capillary plexus (white). The lym- 



