222 ~=George 8. Huntington and Charles F. W. McClure. 
similar relations to the main venous channel and the jugular 
lymph sac, lying ventral to that portion of the latter which is 
derived from the dorsal veno-lymphatic plexus. 
In the course of its development, a large foramen is formed in 
the jugular lymph sac through which the first four spinal nerves 
pass (SP. N.I-IV, figs. 15 and 16). The stages which lead up to 
the establishment of this foramen are shown in figs. 13, 14 and 15. 
At a later stage, the dorsal and ventral portions of the jugular 
sac bounding the foramen, separate anteriorly and then connect 
secondarily with the systemic lymphatics which are formed in- 
dependently of the jugular sac (fig. 17) and not, as claimed by 
Sabin, as the result of a centrifugal growth of the same. 
The fifth spinal nerve (SP.N.V, in figs 13. to 17) also penetrates 
the jugular sac, but through a separate foramen. This condition 
is retained until a late stage of development. 
The sixth spinal nerve (SP.N.VJ) does not penetrate the veno- 
lymphatic plexus nor the jugular sac at any time. From its 
origin in the spinal cord it at first passes ventro-laterad between 
the primitive ulnar and posteardinal veins and contiguous to the 
point where the primitive ulnar vein arches ventrad to open into 
the promontory (figs. 18, 14 and 15). After the primitive ulnar 
vein has given rise to the primitive ulnar lymphatic and has lost 
its connection with the promontory, the sixth spinal nerve then 
passes ventro-laterad and ventral to the primitive ulnar lymphatic 
(figs. 16 and 17). 
III. DETAILED DESCRIPTION OF THE INDIVIDUAL STAGES. 
GENERAL CONSIDERATIONS. 
The jugular lymph sacs of the cat present a considerable range , 
of variation both in the adult structure and in the details of 
their development. This variability appears not only in different 
embryos from the same litter, and possessing approximately the 
same measurements, but even upon opposite sides of the same 
embryo, just as in the adult the right and left sides frequently 
show marked differences in composition and relations in the same 
individual. Moreover, the individual embryos differ greatly in 
