168 OTTO FREDERIC KAMPMEIER 



in mammalian embryos of certain ages. Another anlage (1.2, 

 fig. 8) begins anteriorly as a distinct thickening of the lining of 

 the external jugular (j.d.), but after two or three sections it 

 breaks loose from the vein and proceeds posteriorly as a compact 

 yolk-stuffed endothelial column (1.2, fig. 9) surrounded by mesen- 

 chymal cells. Slightly farther back the same anlage has acquired 

 a lumen. After a variable course, in which it lies occasionally 

 against the venous intima and now and then buds collateral 

 sprouts, it terminates near the posterior niveau of the thyroid 

 anlage, just in front of the heart. 



The earliest initial stages in the genesis of the ventral cephalic 

 lymph sinus occur in 5 mm. embryos. The sketches on plate 2 

 illustrating its inception are representative of the phases observed. 

 Figures 2, 3 and 4 must be considered together since they treat 

 of the same anlage the origin and character of which they plainly 

 depict. In figure 2^ attention should first be directed to the 

 endothelium of the external jugular (j.s.) which is very undulat- 

 ing and nodular, each node or protuberance consisting of a cell 

 and a cluster of yolk corpuscles. Then, one cannot but be 

 impressed by the knoblike appendage (/.) which protrudes from 

 the ventral venous wall into the open tissue reticulum. This 

 endothelial projection is the anterior end of a developing lymphatic 

 channel and, as the figure suggests, arises indisputably by pro- 

 liferation from the lining of the bloodvessel. Its nucleus in the 

 section (fig. 2) does not differ materially from mesenchymal 

 nuclei except for the depressions in its contour which are caused 

 by the crowding of the yolk bodies against it. A large typical 

 stellate tissue cell is shown in the lower left corner of the' picture 

 (fig. 2). Like a number of such cells, its protoplasm at this 

 stage incloses a few scattered yolk granules. Figure 3 represents 

 the next successive section in which the lymphatic anlage (l.) 

 is joined to the vein (j.s.) solely by delicate cytoplasmic threads. 



^ Near the left hand margin of figure 2, the lumen of the external jugular ap- 

 pears to be crossed by an endothelial partition. But this is not the fact. The 

 sketch is of a section taken immediately behind the level in which a medial branch 

 leaves the vein at a very acute angle. Hence the strand of endothelium repre- 

 sents the point at which the walls of trunk and tributary meet, and the two cavities 

 are the lumina of these vessels respectively. 



