ORIGIN OF THE POSTERIOR LYMPH HEART 405 
the latter part of the second week of development, and that 
the spongy tissue thus formed is invaded by capillaries from the 
dorso-lateral branches of the caudal portion of the postcardinal 
veins. The capillaries do not communicate primarily with 
the mesenchymal spaces. Near the close of the third week, 
parallel veno-lymphatic channels are formed in this spongy 
area by the confluence of mesenchymal spaces with one another 
and with the invading capillaries. These veno-lymphatics 
anastomose freely with each other and communicate by two or 
three openings with the veins running along their mesial bor- 
ders. Finally a condensation of mesenchyme and an invasion 
of muscle cells form the wall, while a confluence of the veno- 
lymphatic sinuses gives rise to the single sac-like cavity of the 
adult form of the lymph heart. 
The subject rested at this point until 1912 when E. L. Clark 
(4) cited observations, based on injections, to show that in the 
chick of five days and twenty hours, in the region later occupied 
by the posterior lymph heart, there exists a lymphatic plexus 
connected with the coccygeal veins, but not with the haemal 
capillaries which bear a superficial relation to the lymphatic 
vessels. She also shows that the lymphatic plexus is of a differ- 
ent pattern than the blood capillary plexus and is filled with stag- 
nant blood, which she considers as backed up from the coccyg- 
eal veins. This state of affairs undoubtedly exists in the chick 
of five days and twenty hours but the observation, aside from 
its morphological value, throws no light on the origin and mode 
of growth of the lymphatic plexus. 
E. R. and E. L. Clark (5) in a paper in the same number of 
The Anatomical Record attempt to prove, by observing the 
first appearance and early growth of this blood filled lymphatic 
plexus in the living chick embryo of about five days, that it is 
formed by a purely centrifugal outgrowth from the coccygeal 
veins. To quote from their article: 
The first lymphatics in the tail region of the chick arise as direct 
lateral buds from several of the main dorsal intersegmental coccygeal 
veins, and not by the transformation of a previously functioning 
blood vessel plexus. From now on the lymphatic endothelium is 
