456 



ON THE ORIGIN AND EARLY DEVELOPMENT 



regard to one of the intersegmental coccygeal veins and to the superficial blood- 

 capillaries. When this blood-filled plexus was injected directly with silver nitrate 

 (0.5 per cent) the characteristic endothelial markings appeared, demonstrating that 

 these vessels can not be simple mesenchymal spaces, as might be inferred from Sala's 

 descriptions. 



When the large arteries and veins of the yolk-sac and allantois were opened 

 and the embryo was allowed to bleed freely, the blood could be seen to fade out of 

 the superficial blood-capillaries, but the lymphatic plexus remained undisturbed 

 and still filled with stagnant 

 blood. An embryo thus bled 

 makes a rather striking picture, 

 with the bright-red lymphatic 

 plexus standing out against 

 the white background. Figure 

 22, plate 2, illustrates the super- 

 ficial lymphatic plexus in a 

 6-day chick that had been bled 

 in this manner. 



Examination of the super- 

 ficial plexus in sections of chicks 

 of 5 to 6 days in which the 

 blood-vessels had been com- 

 pletely injected confirmed the 

 observations, made by the 

 other methods, that there is a 

 continuous plexus which is in- 

 dependent of the surrounding 



except for COn- FlQ 1- _p orsal vi ewo f tail r c K ion of an injected embryo of 5 days 23 hours, 

 With branches Of the showing the lymph-heart plexuses on both sides and the coccygeal veins 



with which they are connected, i.c.v. intersegmental coccyueal vrin. 



intersegmental coccygeal x 24. 



veins. In addition, the sections showed that this plexus possesses a definite 

 endothelial lining, and that the endothelial nuclei of the lymphatics have certain 

 morphological characteristics which distinguish them from the nuclei of the ad- 

 jacent connective-tissue cells. These nuclei have a pale, fairly homogeneous, gran- 

 ular appearance, and contain a single nucleolus or a pair of nucleoli, which are 

 definite, discoid bodies, sharply marked off from the remainder of the nuclear 

 material by clear-cut, rounded outlines. The definiteness of this endothelial 

 nucleolus was shown in a striking manner in cases in which an endothelial nucleus 

 had been cut into during the process of sectioning. In many such instances the 

 nucleolus had been dragged out of the nucleus but still retained its characteristic 

 shape. The single nucleolus varies in form according to the shape of the nucleus 

 and to the plane in which the cell has been cut. With the method of staining used 

 these nucleoli have a distinctly reddish color. 



