Development of the Jugular Lymph Sacs. 191 
cation may be found in the adult (jugulo—subclavian angle and 
common jugular angle), or, do the two points of communication 
found in the adult represent primary connections between the 
main venous channels and the veno-lymphatic anlages of the 
jugular sacs which have persisted throughout development? 
There appear to be only two methods of determining these 
questions, namely, by the study and reconstruction of serial sec- 
tions and by the injection of the venous, veno-lymphatic and 
lymphatic vessels in embryos of the appropriate stages. As far 
as our observations on embryos of Felis domestica are concerned, 
we are in the position to make the following definite statement 
which is based on an exhaustive study of numerous series of 
embryos. 
In embryos between 10.7 and 14 mm., inclusive, (crown-rump 
measurement) a very constant and definite picture is offered, in 
strong contrast both with the free veno-lymphatic connections of 
earlier stages, and with the definite and typical taps at the two 
above-defined points which are encountered in the later stages and 
which are present, one or both, in the adult. In this intermediate 
condition, the caudal portion of the emptied sac appears to end 
blindly in two processes (the Jugular and Subclavian Approaches). 
These blind terminals extend to the immediate neighborhood of 
the common jugular and jugulo-subclavian angles, in other words, 
to those points at which in the earlier stages the last of the primi- 
tive connections between the venous system and the lymphatic 
sac persist for a time, and at which in later stages, after the estab- 
lishment of the adult type, the wedge-shaped lymphatico-venous 
entrance with valve formation is found. But, in the intermediate 
stages now under consideration, the most careful scrutiny under 
high power, of excellently fixed and stained series in large number, 
fails to reveal any definite continuity whatever between the lymph 
sac and the vein. We have here, therefore, a positive and appar- 
ently uniform observation which, in judging the general question 
at issue, cannot be disregarded, since one well-established fact 
is of more value than any number of conjectures. This fact we 
regard as definitely determined, within the natural limits which 
are imposed by errors in observation and interpretation of a 
