ORIGIN OF LYMPHATICS IN BUFO 169 



Three sections back, the diameter of the external jugular (j.s., 

 fig. 4) has markedly contracted while that of the incipient lymph 

 channel has increased. This rudiment is solid from end to end, 

 being stuffed with yolk, which attribute sharply demarcates it 

 from the embryonic connective tissue. To repeat what has 

 been said but a moment ago, the visible differences between 

 lymphatic endothelial nuclei and mesenchymal nuclei are too 

 trivial at this early genetic period to warrant our emphasizing 

 them as positive differential characteristics. 



One of the earliest sinus anlagen observed by the writer is 

 shown in section in figures 5 and 6. It is of very brief extent, 

 being only as long as the total thickness of four or five sections. 

 In the first (fig. 5) of these two sketches it appears as a compact 

 protuberance (l.) on the intima of the right external jugular 

 ij.d.). The structure labelled h.c. in the same drawing and which 

 at first glance might be mistaken for another initial lymphatic 

 is a blood cell closely pressed against the lining of the vein. In 

 very young toad larvae blood cells like vascular walls are abund- 

 antly supplied with yolk. Overlooking one section we come to 

 figure 6. Here two features should be noted in particular; 

 firstly, the blind slit-like space within the lymphatic anlage, 

 and secondly, the appearance of a boundary between anlage 

 and vein which however is still imperfect since the line of division 

 extends only partway. The cavity of the anlage is not in con- 

 nection with that of the vein. 



From the data so clearly displayed in the camera lucida sketches 

 reinforced by much similar evidence at the writer's command, 

 the following generalization or coherent account of the genesis 

 of the ventral cephalic lymph sinus can be constructed. In 5 mm. 

 embryos, a stage in which the mesenchyme and the vascular 

 endothelia of the head already differ to a noteworthy degree in 

 the fact that the former is more meagerly furnished with yolk, 

 knot-like thickenings occur at unequal intervals on the lining of 

 the external jugular veins in the direction of their long axis. 

 These thickenings, potentially lymphatics, are few in number and 

 arise unquestionably as proliferations of the venous intimal cells 

 and like them are provided with many yolk spherules which are 



