432 RANDOLPH WEST 



As this investigation has been concerned solely with the 

 origin of the lymphatic plexus which later forms the lymph heart, 

 the later history of the lymph heart has not been studied. A 

 cursor}^ examination of a 16, 16.5, 17 and 18 mm. embryo would 

 indicate that the conclusions of Sala are in. the main correct, 

 and that the plexus coalesces to form the single cavity of the 

 lymph heart. The formation of the musculature, valves and 

 the number of venous taps in the stages later than 15 mm. has 

 not been studied. 



GENERAL DISCUSSION 



Of the previous investigators of the posterior lymph heart 

 in the chick, Sala (1) and Mierzejewski (2) have not committed 

 themselves as to the origin of the lymphatic endothelium, while 

 E. R. and E. L. Clark hold that the lymphatics are outgrowths 

 from the veins and that the endothelium is specific. E. R. and 

 E. L. Clark (5) have studied the growth of the lymphatic plexus 

 in the living chick under the binocular microscope, using the 

 stagnant blood backed up in the growing lymphatics from the 

 veins as the index to lymphatic growth. To quote from their 

 paper: 



Since stagnant blood in the interior of the lymphatics is the index 

 on which these studies are based, it was important to determine whether 

 the blood always fi.lls the lymphatics to their tips. This was tested 

 in two ways, by pressure over the part filled with blood to see if it 

 could be forced farther; and by injection. As a result of numerous 

 tests by both of these methods it was found that in these early stages, 

 practically all of the lymphatics, save very fine connections are filled 

 with blood. * * * * Hence, since the blood fills the successive 

 extensions of the lymphatic as soon as formed, the use of the stagnant 

 blood as an index for the study of lymphatic development is justifiable 

 (p. 255). 



This method has overlooked even the possibility of the presence 

 of disconnected mesenchymal spaces entering into the formation 

 of this lymphatic plexus. How, extravasation excepted, could 

 pressure over the blood-filled plexus or injections into it, reveal 

 disconnected lymphatic anlagen in the form of blind spaces in 

 the mesenchyme? It is" obviously impossible to detect small 



