OF THE LYMPHATIC SYSTEM OF THE CHICK. 461 



veins. When the sections of the embryo were studied with the higher magnifica- 

 t : ons a continuous plexus was reconstructed in this region. Some of the vessels 

 are as large as blood-capillaries and contain blood-cells; these undoubtedly repre- 

 sent the blood-filled knobs seen in the living; others are much narrower, and still 

 others are solid. Long narrow processes extend superficially and ventrally from 

 this plexus in the exact position in which the blood-filled sprouts make their appear- 

 ance in living chicks a few hours later. The plexus connects at several points with 

 branches of the intersegmental coccygeal veins in each segment (X in the drawing) . 

 Some of these connections arc fairly large and others are solid. 



This lymphatic plexus is composed at this early stage of continuous endothe- 

 lium which has the characteristic endothelial nuclei already described. Figures 26 

 and 27, plate 4, were drawn from vessels composing this same plexus and show the 

 differences in form and color between the nuclei and their nucleoli and those of 

 adjacent connective-tissue cells. Figure 29, plate 4, is also from this plexus and 

 shows a lymphatic endothelial cell in mitosis. Figure 28, plate 4, is from another 

 specimen of the same stage and shows one of the early blood-filled knobs in section, 

 with its connections with the veins. One of these connections is open and contains 

 a blood-cell, while the other is solid and thread-like. 



Just as the study of the living blood-filled lymphatic plexus in chicks of 5 to 

 6 days showed that the reconstruction from cross-sections gave only a very incom- 

 plete account of the true conditions (F. T. Lewis, 1906, and Miller, 1912), so the 

 picture obtained from these studies, made with the oil-immersion, shows that our 

 earlier account of the blood-filled "bunches" seen in the living and injected at this 

 stage (Clark and Clark, 1912) was extremely fragmentary. This is also !he stage 

 which West (1915) describes as his first stage (a chick of the same age as this speci- 

 men, and one measuring 10.5 mm. after fixation and dehydration), in which only a 

 few "mesenchymal spaces" could be reconstructed in this region. Figure 31, plate 

 6, represents the condition in a part of the region in a still younger embryo. This 

 chick was 4 days and 9 hours old and measured 11.75 mm. in the fresh state. No 

 blood-filled lymphatics could be seen in the living chick. The blood-vessels were 

 completely injected with India ink and the embryo fixed, sectioned, and stained in 

 the usual manner. The oil-immersion reconstruction showed us that even at this 

 stage a plexus of vessels, with the characteristic form of an early lymphatic plexus 

 and the characteristic endothelial nuclei, is present in the same region which is later 

 occupied by the blood-filled lymphatic plexus, and still later by the beating lymph- 

 heart. The plexus at this stage is not so luxuriant as that of the next older stage 

 shown in figure 30, plate 5, and the vessels composing it are relatively smaller. The 

 fine and solid connections between different parts of the plexus are relatively more 

 numerous. A number of venous connections are also present at this early stage. 

 Finally, figure 32, plate 6, is taken from an oil-immersion reconstruction of a por- 

 tion of this region in a chick of 4 days and 7 hours, measuring 9 mm. in the 

 living. Here, besides the injected blood-vessels and the indifferent mesenchyme, 

 there were suspicious-looking vessels with characteristic endothelial nuclei in a part 

 of the region chosen for intensive study. As in the other sectioned specimens, all 

 of these nuclei were drawn, with the characteristic nucleoli, as was also the endo- 



