Development of the Jugular Lymph Sacs. 241 
As will be seen by comparison with the next series 138 (fig. 35), 
tributary 4 on the right side of the embryo now under considera- 
tion represents a stage in which the capillary plexus surrounding 
its terminal has not yet been clearly defined into somatic and 
veno-lymphatic components. A comparison of this reconstruc- 
tion with the one shown in fig. 35 may, however, justify the 
inference that the irregular, bifurcated dorsal portion of tributary 
4 (4Vl, fig. 31) may represent a veno-lymphatic anlage, while 
the dorsal somatic component (4S in fig. 31,) in this case would 
separate subsequently from the veno-lymphatic component of 
tributary 4 and from the para-precardinal channel, and gain an 
independent entrance into the dorso-medial aspect of the promon- 
tory. This is one of the instances, so frequently encountered in 
the course of this investigation, in which it is evident that the 
embryo in question has been fixed during a period of transition 
from one well-defined stage to the succeeding condition. If we 
were basing our conclusions on a small number of sectioned and 
reconstructed embryos, it would be necessary to record the 
observed conditions without venturing a positive interpretation. 
Our material, however, has been so extensive and conclusive, and 
so representative of each important ontogenetic phase, that we 
feel justified in foretelling the significance of conditions which, in 
a given embryo, unmistakably point toward the stage, which, 
without interruption, would have been attained. 
We may be pardoned if, in connection with the series under dis- 
cussion, we venture to emphasize the importance of embryological 
investigations based on the careful analysis of a large number of em- 
bryos of approximately the same age. Our previous experience in 
the study of the embryonic venous variations of the cat has proved 
conclusively to us that it is necessary to determine normal onto- 
genetic conditions for each main developmental period, and that 
the question of embryonic departures from this norm equals 
in importance and significance the value which should be assigned 
to variations of the adult vascular system. 
On the left side of this embryo (fig. 832) the ventral curvature of 
the cephalic arch is well established. 
A-B enters as a large single vessel at the junction of the cephalic 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2. 
