Development of the Jugular Lymph Saes. 187 
mental period in which we may regard these venous derivatives as 
having assumed a lymphatic significance. Is it at a time when 
the adult condition has been reached and the lymphatic system 
begins to perform its destined function? Is it at a time, as in 
Lewis’ 11 mm. rabbit (14 days) in which the venous anlages have 
fused into two large sacs, containing no blood corpuscles, which, 
in virtue of their complete separation from the main venous 
channels, cannot, in the generally accepted sense, function in their 
present condition in the lymphatic organization of the body? Is 
it at a time, as in Lewis’ 10 mm. rabbit (14 days), in which his 
chain of lymphatics, constituting the jugular lymph sac, contains 
many blood corpuscles, and possibly communicates with the veins? 
Or, finally, is it at a time, as in the 8.5 mm. cat and in stages 
which precede it, in which the anlages of the jugular sacs are in 
the process of being separated from the venous channels but are 
still in communication with them? 
The development of the jugular lymph sacs is not a haphazard 
process, and their anlages, although derived from the embryonic 
veins, are as definite in their character and mode of development 
as those of other organs. If we are able to draw a distinction be- 
tween thymus and pharyngeal tissue at any time before the thy- 
mus anlages have separated from the pharynx and become trans- 
formed into a functional body, we are likewise justified in draw- 
ing a similar distinction between the lymphatic and venous 
structures. It is not the purpose of this paper to enter further 
into a discussion of this question of specificity of tissues, but 
merely to point out that the claim, made by Dr. Sabin and others, 
that no lymphatics make their appearance in the body before the 
jugular sacs are formed, can best be answered by asking for the 
definition of a lymphatic. 
In a former contribution to this subject, the writers applied the 
term “Veno-lymphatics” to all the venous anlages of the jugular 
sacs, at a time when these anlages were filled with blood and in 
free communication with the venous channels. In early stages 
the only structural distinction that can be made between the 
venous anlages (veno-lymphatics) of the jugular sacs and the 
fully formed sacs themselves, is that the former, being in com- 
