OF THE LYMPHATIC SYSTEM OF THE CHICK. 457 



The nucleus of the mesenchyme cell differs in all the particulars mentioned. 

 It contains two or more nucleoli which are not sharply differentiated from the 

 remainder of the chromatin material of the nucleus, but which extend out into 

 prongs and threads, and these do not have a characteristic shape. These nucleoli 

 take a distinctly bluish stain. The remainder of the nucleus of the connective-tissue 

 cells is darker in appearance than that of the endothelial cells and frequently contains 

 small clumps of chromatin material. In chicks of this stage the blood-vessel endo- 

 thelial nuclei are slightly smaller than those of the lymphatics, more regular in 

 shape, and are apt to contain two nucleoli instead of a single large one. In earlier 

 stages these distinctions are frequently absent, and without the injection material 

 present in the blood capillaries it would be practically impossible to distinguish the 

 two types of nuclei. Figures 26 and 27, plate 4, show microscopic drawings of a 

 younger embryo (4 days and 23 hours) and illustrate the difference between the 

 endothelial nucleus and the nucleus of the mesenchyme cell. 



An examination of sections confirmed the observations based upon injections 

 and the study of the living; {. e., that in chicks of 5 to 6 days a primitive plexus of 

 lymphatics is present in this posterior part of the pelvis and in the tail region and 

 precedes the formation of the lymph-ducts and the beating lymph-heart which 

 occupy the same area in older embryos. The plexus is indifferent and irregular in 

 character; comparatively large bulbous nodes alternate with very fine, even solid 

 connections and delicate processes. The connections of this plexus with the first 

 five intersegmental coccygeal veins or their branches were easily seen in sections, 

 but aside from these the plexus was found to be independent of the blood vascular 

 system. A number of mitotic figures were seen in the endothelium of this lymphatic 

 plexus. The presence of these mitoses, scattered here and there in endothelial 

 cells, as well as the general character of the plexus, gives the impression of a wild 

 and rapid growth. 



In studying sections, cut parallel to the surface, of chicks of 5 to 5| days in 

 which the blood-vessels had been completely injected, with the aid of the oil- 

 immersion lens a more extensive plexus of lymphatics could be made out than we 

 had been able to discover with other methods. For example, figure 3 is from a 

 reconstruction of a portion of the plexus over the pelvis of a chick of 5 days 7| 

 hours. In this specimen the blood-filled lymphatics of the posterior part of the 

 pelvis and of the lymph-heart region were injected with India ink and the blood- 

 vessels subsequently filled with Berlin blue gelatin (5 per cent) . The drawing shows 

 a part of the rich plexus of fine, delicate vessels which we were able to reconstruct 

 in an area well beyond the injected part. The reason why the injection failed to 

 reveal this is obvious from the character of the plexus. Although the endothelium 

 is continuous the lumen is not. Many of the connections are exceedingly narrow 

 and others are entirely solid. It is interesting to note the large number of mitoses 

 present in this small part of the plexus. 



As we stated in 1912 and 1915, the stagnant blood normally present in the 

 early lymphatics of this stage is due merely to pressure conditions. It is easy to 

 show by injection experiments on living chicks, by cleared injected specimens, and 



