96 The Development of the Lymphatic System in Rabbits 
The second theory, that of the gland-hke origin of the lymphatic system, 
is supported by the remarkable injections of pig embryos, made by 
Prof. Sabin.” She considers that in mammals, this system buds from the 
venous endothelium at four points, forming four lymphatic ducts. The 
ducts are dilated to form four lymph hearts, which, though destitute of 
muscles, correspond with the four lymph hearts of amphibia. Starting 
from these hearts, lymphatic outgrowths invade the body, and those from 
the anterior pair unite with those from the posterior pair. Then the 
posterior hearts lose their original openings into the veins, but those of 
the anterior hearts persist as the outlets for the thoracic and right lymph- 
atic ducts respectively. The lymph hearts themselves are said to be- 
come transformed into lymph nodes (05, p. 355). 
According to this idea, the lymphatic vessels are true lymphatics from 
their earliest inception. 'They differ from other branches of the veins 
by their very oblique angle of entrance, and by failing to anastomose with 
arteries or veins. Anastomoses with other lymphatics are abundant, due 
to absorption of contiguous walls (Ranvier, 97, p. 74). 
The supposition suggested by the study of the subcardinal veins is 
intermediate between those of Sabin and Sala. The endothelium of 
the lymphatics is considered to be a derivative of that which lines the 
veins, since the lymphatics are at first a part of the venous system; but 
by becoming detached from their origins these lymphatics form closed 
sacs in the mesenchyma. Later they acquire permanent openings into 
the veins, and many connections with other lymphatics. 
In studying the development of the lymphatic vessels, several methods 
have been employed. Sala used serial sections, generally of injected 
embryos, and made wax reconstructions of the posterior hearts. Sabin 
perfected the method of injection which had been employed by Ranvier 
for pigs of 100 mm., so that it was applicable to those of 20 mm. By 
this means she studied the large jugular lymph sacs, or “ anterior hearts,” 
which, as Saxer discovered (p. 370), are the earliest lymphatic vessels 
to appear. On the basis of injections she was enabled to present the 
first connected account of the development of the mammalian lymphatic 
system. This was illustrated by a series of conventional diagrams, in 
which the blood-vessels are shown without details. Thus the internal 
2 Ranvier described the interesting analogies, both functional and embry- 
ological, between typical glands and the lymphatic system. Sabin does not 
adopt the idea that the whole lymphatic system represents a few large 
glands. She does, however, describe it as arising from four blind epithelial 
(endothelial) outpocketings which ramify in the connective tissue, and this 
origin may be designated, after Ranvier, as “ gland-like.”’ 
