ORIGIN OF LYMPHATICS IN BUFO 165 



that of a fairly definitive stage in the organization of the sinus 

 and then treat of progressively earlier conditions, which may 

 subsequently be summarized in a more logical manner. 



In figure 1 the ventral cephalic lymph sinus (L) of an 11 mm. 

 embryo is shown in section as a wide clear chamber, the periphery 

 of which is sharply defined and the cavity is not interrupted by 

 partitions. Disregarding the disparity in magnifications, with 

 it should be compared the one represented in figure 13, from a 

 9 mm. embryo, where it (l.) is seen to be less broad dorso-ven- 

 trally and to be crossed wholly or part way by tissue strands (t. 

 and s.). Imagined in its entirety, these trabeculae are very 

 numerous and exhibit several variations. Some are proportion- 

 ately thick (t., figs. 13 and 14) and are composed of a core of 

 mesenchyme (m.) covered with endothelium; others are more 

 tenuous, barely stretching across the lumen; and still others 

 exist as spurs (s.) of varying lengths which project into it from 

 the surface. Furthermore, there are scattered freely through- 

 out the cavity small clumps of lightly staining debris (d.) whose 

 appearance would suggest their being cellular shreds or frag- 

 ments of former trabeculae or partitions, which had perchance 

 become separated from the walls of the sinus during its formation. 

 Such an explanation presents itself as the most credible one. 

 Pictured in figure 14 individual yolk spherules are conspicuous 

 here and there in the endothelial cytoplasm generally in the 

 neighborhood of a nucleus. At this stage the lining cells also 

 possess all of the intrinsic qualities of typical endothelia and 

 accordingly it is an easy matter to distinguish them from the 

 stellate mesenchymal cells. 



Figure 12 delineates a little more than the right half of a section 

 through the sinus (L) in an 8 mm. embryo and plainly indicates 

 its plexiform character at this period. This is confirmed by an 

 enlarged graphic reconstruction^ which shows it to be a network 

 of interanastomosing vessels arranged one layer deep in a slightly 

 curved plane, and which, viewed from the ventral surface, brings 

 to mind a coarsely and irregularly meshed sieve. Followed 



* Wax models and graphic reconstructions of crucial stages in the development 

 of the lymph hearts, ducts and sinuses of the toad will be pictured in the later 

 contribution. 



