OF THE LYMPHATIC SYSTEM OF THE CHICK. 455 



tip of the pelvis. In these younger embryos the lymphatic plexus, as seen in the 

 living or injected specimens, is less luxuriant than in the chicks of 5^ to 6 days, and 

 the vessels composing it are much finer. 



The appearance of the posterior blood-filled lymphatic plexus in the living 

 chick is illustrated in plate 1, figure 17. The more superficial portion, over the tip 

 of the pelvis, connects with the deeper plexus which later forms the lymph-heart. 

 This superficial plexus is easily distinguishable from the blood-capillaries of the 

 region in a number of ways. For the most part it lies beneath the superficial blood- 

 capillaries. The pattern of the two sets of vessels is different; the lymphatic 

 capillaries are more irregular in form than the blood-vessels. The dark-red color 

 of the plexus containing stagnant blood contrasts with the lighter, more yellowish 

 tinge of the blood-capillaries. The most striking feature is the contrast between 

 the rapid motion of the blood-corpuscles in the blood-capillaries and the blood in 

 the lymphatic plexus which remains stagnant. Observation of the two plexuses 

 shows clearly that this plexus of capillaries filled with stagnant blood is a distinct 

 and independent system of vessels. 



Various tests were also made in order to learn more of the character of this 

 early plexus and its relation to the blood vascular system. The plexus containing 

 stagnant blood was injected by direct puncture of a selected lymphatic capillary. 

 The near-by circulating blood-vessels remained undisturbed after such an injection, 

 thus showing the independence of the two systems. Figure 21, plate 2, illustrates 

 such an injection in the exact location occupied by the beating lymph-heart of older 

 embryos and a few delicate superficial vessels continuous with it. The injection 

 often reveals a few fine connections which had not been observed in the living, but 

 in general the extent of the plexus, as demonstrated by the presence of the blood and 

 by injection, is the same. 



When a small amount of ink was injected into one of the vessels of this lymph- 

 heart plexus without disturbing any of the superficial blood-capillaries, the granules 

 could be seen to enter the intersegmental coccygeal veins and to move along in the 

 main caudal vein. Cleared specimens with injected lymphatics enabled us to 

 study this relationship of the veins still further. Figure 1 shows the lymph-heart 

 plexus and its venous connections of both sides. The view is a dorsal one, for with 

 the chick in its natural position, lying on one side, the lymph-heart plexus lies 

 directly over the intersegmental veins of the tail, thus concealing the points of 

 connection from the observer. 



Complete blood-vessel injections were obtained, and such an injection left 

 the lymphatic plexus filled with stagnant blood, while all the surrounding blood- 

 capillaries became filled with the injection mass. Figure 18, plate 1, is a drawing 

 of a fresh specimen made immediately after such a blood-vessel injection. 



Double injections were obtained with the blood-vessels completely injected 

 with India ink and the lymphatics filled with silver nitrate or Berlin blue. Plate 

 3, figure 24, shows such an embryo with some early lymphatics in the lymph-heart 

 region injected with silver nitrate. A thick cross-section made from the same 

 specimen (plate 3, fig. 25) shows the location of some of these little vessels with 



