410 ELEANOR LINTON CLARK 



phatics, was found to be present in the axillary region, which 

 •connected the side plexus wdth the deep jugular plexus. All 

 the other superficial lymphatics still retained the irregular 

 indifferent form of the primary plexus. 



In testing many embryos of this stage, I found a closely graded 

 series of chicks which showed a progressive extension of the area 

 of lymph-flow. First, the granules circulated only in the region 

 just posterior to the shoulder, then they moved anteriorly for 

 about half the distance between shoulder and leg, and later for 

 the entire area between the two limbs, and, in the oldest embryos 

 of this stage, granules injected into the lymph vessels on the an- 

 terior part of the pelvis moved anteriorly from the point of 

 injection until they disappeared beneath the shoulder. In each 

 of these embryos, after injection, a definite channel was present 

 in a position exactly corresponding to the path followed by the 

 circulating granules. In the case of the older chicks in which 

 the lymph-flow had been present throughout the side region and 

 the movement of granules was more direct and rapid than in 

 the younger embryos, injection showed that the channel was 

 wider and straighter, as well as longer. During the latter part 

 of this stage, a new plexus of lymphatics grows to the surface 

 from the deep jugular plexus. It emerges in front of the shoulder 

 and grows around dorsally to the shoulder. 



It appears, therefore, that the earliest lymph-flow, in the 

 superficial lymphatics, starts in the region between the shoulder 

 and leg and that its direction is anterior, into the anterior and 

 posterior cardinal veins. Associated with this first lymph flow, 

 a channel, the first of the superficial lymph trunks, differentiates 

 out of the irregular primitive plexus in this region. During this 

 stage the blood is washed out of the anterior lymphatics but still 

 remains in the posterior plexus of the pelvis and tail which re- 

 tains its indifferent character. 



It is probable that the pressure in the tail veins, with which 

 the posterior lymphatic plexus connects, is higher and hence 

 more difficult to overcome, than that of the anterior and posterior 

 cardinal veins into which the anterior lymphatics drain, and that 

 to this is due the anterior direction of the earliest lymph-flow. 



