ORIGIN OF LYMPHATICS IN BUFO 175 



strangely enough it squeezes between vein (i.v.) and myotome 

 (m.s.). Three sections caudad an endothehal nucleus curiously 

 protrudes into the lumen of the lymphatic (fig. 23). Seven 

 sections beyond this level the anlage has become compressed and 

 in transverse section appears as a crescent shaped cavity {L, 

 fig. 24) clinging to the venous lining. Three additional sections 

 bring us just beyond its end (fig. 25) ; in fact, it terminates at the 

 large endothelial nucleus on the ventral venous wall. In this 

 (fig. 25) and the following levels there is no indication of the lym- 

 phatic anlage. Between it and the anterior lymph sac one 

 hundred and sixty sections intervene, but in this interval two 

 similar anlagen occur which, however, are of much shorter length. 



In another younger 6 mm. embryo the longitudinal venous 

 channel of the same locality, in which the anlage just discussed 

 has its being, pursues its way alone and without a dependent 

 structure adhering to its walls, save for a distance of three sec- 

 tions where a small lymphatic rudiment (l.) is intimately associ- 

 ated with it (i.v.), as indicated in the sketch, figure 26. Its 

 appearance brings to mind some of the initial rudiments of the 

 cephalic lymph sinus, for instance, one (^.3) shown in figure 9. 



In the foregoing description of two genetic stages of the supe- 

 rior lateral lymph duct no mention was made of the mesenchyme. 

 It requires but cursory notice, for in the youngest toad larvae 

 it is extremely sparse between trunk myotomes and epidermis; 

 only after the larvae have attained the length of 8 mm. does the 

 territory between these two structures expand and the mesen- 

 chyme invade it more plentifully. On the other hand, in the 

 area in which the formation of the inferior lateral lymph duct 

 takes place, that is lateral to the Wolffian duct and the post- 

 cardinal, the mesenchyme is considerable even in 6 and 7 mm. 

 embryos. Moreover, it loses its yolk content much later than 

 does that in the head; yolk bodies may be found in its cells long 

 after the rudiments of the lymphatic duct had their inception. 

 Thus it is evident that, though the trunk mesenchyme pro- 

 portionately contains less yolk than do the endothelia, the pres- 

 ence or absence of this substance can not strictly be used as a 

 differential character in the development of the l3rmph duct 



