Development of the Jugular Lymph Saces. 228 
a chronological sense. In some, one portion of the veno-lym- 
phatic system is well developed, while the remainder is retarded. 
Ultimately, within certain limits, all embryos develop congruent 
conditions, but owing to this variability as to the coincidence 
of all the factors at a given period, it has been found necessary to 
examine a number of embryos of about the same age, and to group 
the vessels thus obtained in certain cases into a composite picture 
which will serve as a guide for the proper interpretation of each 
individual embryo falling within a given period, although all of 
the developmental possibilities of this period may not be pre- 
sented by each and every individual embryo in the series. Sucha 
series of composite pictures has already been discussed in con- 
nection with the preceding topic (figs. 8 to 21 inclusive.) 
Of course, in addition to this general source of variability, 
individual embryos present a certain range of personal and acci- 
dental variation in development. Not only will individual 
embryos show advanced development in certain areas, and retarded 
conditions in others, but they also differ in the minor details, 
such as in the number and arrangement of secondary tributaries 
and their anastomotic conditions. 
In order to facilitate the interpretation of the individual reconstruc- 
tions figured in this paper, the reader is advised to refer each one to 
the series of diagrams already described in the preceding pages 
(figs. 8 to 21, inel.). 
In the figures of the individual reconstructions the approxi- 
mate derivation of the components of the jugular lymph sac 
is indicated by the same color scheme as that adopted in the 
introductory analysis for the diagrams, figs. 8 to 21, inclusive, so 
that comparison of any given series with the corresponding 
generalized developmental stage described in the analysis can 
readily be made. 
We have adopted this plan in order to emphasize the uniform 
genetic type of lymph sac development in an extensive series of 
embryos presenting the inevitable individual variations. Of 
course the value assigned by us in this presentation to the indi- 
vidual components in a given series is a question of personal inter- 
pretation of the actual conditions offered by the embryo. This 
