20 FATE OF PRIMARY LYMPH-SACS IN ABDOMINAL REGION OF PIG, ETC. 



capillaries gradually invade the body in orderly sequence in definite and character- 

 istic zones and layers. The growth is always in the capillary bed that is, all 

 lymphatics develop as capillaries, and the earliest ones to develop become the first 

 lymph trunks or ducts." To make the story more complete (at least in the pig), in 

 the next stage these lymph-sacs break up and are converted into a coarse plexus 

 of lymph-channels, which in turn are transformed into lymph-nodes or glands. 



As to the secondary lymphatic system that is, the peripheral vessels the de- 

 velopment of those of the skin (Sabin, 1904), small intestine (Heuer, 1909), lung (Cun- 

 ningham, 1916), heart and stomach (Cash, 1917, 1921) has already been studied. 



The work of A. H. Clark (1912-13) on the fate of the jugular lymph-sacs gives 

 a general survey of the lymphatics of the anterior region of the body in the pig and 

 shows how the lymphatic channels of the head, neck, and thorax are related to the 

 primary lymph-glands. In the present paper it is desired to give a similar survey of 

 the lymphatics in the abdominal and pelvic regions and to determine more 

 definitely the fate of the primary sacs of the abdominal region. No attempt will be 

 made to present in detail the development of the lymphatics of the different organs. 



Throughout the investigation the encouragement and assistance of Dr. 

 Florence R. Sabin have been a constant help, and my sincere appreciation and 

 thanks are due to her. 



METHOD. 



The material consisted of living pig embryos varying in length from 4 to 20 

 cm. The lymphatics were injected and the specimens cleared by the Spalteholz 

 method as described by Sabin (1915). Silver nitrate (2 per cent) and india-ink 

 were used for injections. With the latter the injections were nearly complete. 

 Some were made through the retroperitoneal sac, but more thorough injections of 

 the abdominal and pelvic regions were secured by introducing the needle into the 

 thoracic duct on the left side, a short distance posterior to and behind the arch of 

 the aorta, as described by Heuer (1909). 



At about the stage of 7 cm., valves are being formed in the lymphatic vessels 

 and the primary sacs no longer remain homogeneous, but begin to break up into 

 plexuses of vessels which are the forerunners of the primary lymph-glands. For this 

 reason, beyond the stage of 8 cm., when it was difficult to get the injection mass to 

 pass the valves, the different organs were injected directly and the lymphatic 

 drainage to the several glands was thus indicated. 



In the pig the origin of the lymphatics of the abdominal and pelvic regions can 

 be traced to lymphatics arising ventral to the aorta (fig. 1) and dorso-lateral to the 

 aorta (fig. 4). With the growth of the embryo and the beginning formation of the 

 lymph-glands this distinction can not be made out so readily, since in both cases the 

 glands tend to lie lateral to the aorta. When glands are definitely formed, those 

 arising from lymphatics dorso-lateral to the aorta in the main lie with those from 

 the ventral side. 



