Development of the Jugular Lymph Sacs. 179 
accords with the observation that they first appear in rabbits of 9.5 to 
10 mm., but does not agree with Ingalls’ opinion that in a 4.9 mm. 
human embryo certain vessels represent the first anlage, or earliest fore- 
runners, perhaps, of the lymphatic system in man. The vessels in cues- 
tion are clearly veins. 
Unless the conditions in man differ widely from those in the cat, 
it is evident to the present writers that Dr. Lewis has not as yet 
identified the earliest venous anlages or forerunners of the jugular 
lymph sac in human embryos, but has observed only the deriva- 
tives of these anlages after they have become distinctly lymphatic 
in character and when, by themselves, at this stage of development, 
they present no direct evidence that they have been derived 
from the veins. 
In the course of our work on this subject, which has extended 
over a period of four years, we have been constantly surprised by 
the extraordinary variability and complexity which characterize 
the detailed development of the jugular lymph sacs, and it has 
only been possible to interpret these conditions after the study of 
avery complete series of embryos, including a number of em- 
bryos of approximately the same ages. It has also been neces- 
sary to consider, in connection with the embryonic history of the 
jugular lymph sacs, these structures and their variations in the 
adult. 
The primary principles underlying the development of the 
jugular lymph sacs are (1) the development of a secondary chan- 
nel parallel to the embryonic precardinal and the Cuvierian end of 
the posteardinal; (2) the association with this secondary channel 
of a certain number of dorsal precardinal tributaries, and (3) the 
separation of these two sets of venous elements, which we have 
termed “‘Veno-lymphaties,”’ from the main venous channels and 
their subsequent conversion into the definite jugular lymph sacs 
by a process of growth and fusion. 
Although a general principle of development has not been 
difficult to establish, it has proved, in some cases, a matter of the 
greatest difficulty to determine the actual mode of origin of cer- 
tain of the venous anlages (veno-lymphaties) of the jugular lymph 
sacs owing to the variable manner in which these veno-lymphatics 
