Frederic T. Lewis 101 
ee 
‘like the veins, they are deseribed and figured as 
suggesting the lymph hearts of the chick (p. 238). It is stated that 
spaces in the mesentery ” 
these spaces “ may be subcardinal derivatives.” Re-examination of this 
embryo has yielded no more definite information. The spaces which are 
undoubtedly lymphatic, as shown by their later development, seem to re- 
place veins of the preceding stage. In the same way the lymphatic 
vessels in the mesentery, accompanying the superior mesenteric and: the 
gastric veins may have arisen as the branches of those vessels seen in 
Fig. 3. They extend around the superior mesenteric artery, which the 
corresponding vein accompanies. The fused vitelline vein is destitute of 
small branches, and is not provided with lymphatics. 
The jugular lymph sac in Fig. 4 has completely surrounded the third 
and fourth cervical nerves. It envelops two-thirds of the circumference 
of the internal jugular vein. On the right side of the embryo, in one 
section (No. 476), a minute orifice connected the sac and the vein. 
It was not in the position of the adult opening between these structures, 
and was not matched on the opposite side. The deep subcutaneous out- 
growth from the jugular sac has become greatly dilated in its distal portion. 
Near the beginning of the external mammary vein, a large lymph space 
is found wedged between two converging venous branches. This space is 
not connected with the veins. It may be a remnant of the lymphatic 
vessels which in the preceding stage accompanied the dorsal root of the 
ulnar vein. A few slender detached lymphatics follow the external 
mammary vein. Finally there are two lymphatics which appear to have 
arisen from branches of the azygos vein, one near the vagus nerve (Fig. 
4, x) and the other along the aorta (Fig. 4, y). ‘The former connects 
with a small vein, the latter ends blindly not far from one. Obviously 
when a connection with a vein is well preserved the structure in question 
would be considered a venous branch; and after becoming detached, were 
it not for its endothelial wall, it might be called a mesenchymal exca- 
vation. The study of this and the following specimens seems to show that 
the lymphatics along the aorta (thoracic ducts) are derived in part from 
the azygos veins; below, from the subcardinals; and above, from the 
jugular sacs. 
In order to determine whether the lymphatic system of the rabbit 
differed materially from that of other mammals, reconstructions were 
made of a 21 mm. pig, and a 15 mm. cat. The former is of special 
interest as a basis of comparison between the present work and.that of 
Prof. Sabin. The lymphatics in the pig (Fig. 5) consist of a pair of 
jugular lymph sacs, a pair of subcardinal sacs which fuse with one 
another irregularly and are variously subdivided by thin septa, and 
