ORIGIN OF LYMPHATICS IN BUFO 173 



To forestall the criticism of insufficient data, a few typical and 

 decisive stages in the genesis of two other lymphatic vessels will 

 be briefly detailed. These stages shall primarily show the occur- 

 rence of discontinuity in a developing Ijmaph duct just as the 

 developing sinus has shown in the first instance the origin of its 

 endothelium from venous intima. The channels to be considered 

 are the lymph ducts situated laterally, a pair on each side, and 

 extended through the entire length of tail and trunk to open into 

 the anterior lymph hearts (c/. Hoyer's fig. 417, 7th edition, 

 Wiederscheim's "Vergleichende Anatomie der Wirbeltiere '). 

 For a more precise conception of their place in the anatomy 

 of the embryo the reader is directed to figure 15, which pictures 

 a little more than the sinistro-dorsal quarter of a section through 

 the mid-trunk region of a 9 mm. embryo. The lymph ducts, 

 l.s. and Li., are seen to be located between epidermis and myo- 

 tome {m.s.), the first (l.s.) near the superior or dorsal border of 

 this structure, and the second (Li.) near the inferior or ventral 

 border. To make more comprehensive the succeeding sketches, 

 the relative position and direction of two or three neighboring 

 organs should be pointed out. The Wolffian or pronephric 

 duct (w.) is intercalated between the lower margin of the muscle 

 segment and the dorsal peritoneum or roof of the coelom. Along 

 its free walls run two veins which in reality are the medial and 

 lateral divisions of but one, the postcardinal. Regarded in their 

 longitudinal aspect, the postcardinal divisions are bound to- 

 gether by numerous cross-anastomoses, which pass over and 

 under the pronephric duct at unequal but frequent intervals and 

 hence produce a cylindrical vascular network which closely in- 

 vests this duct.*' The postcardinal vein of each side also give 

 off m regular sequence the intersegmental veins (i.v.), every one 

 of which supphes chiefly a myotome. 



" According to Goette ('75) the postcardinal veins in Anura lie medial to the 

 pronephric ducts. But the writer can not subscribe to this statement uncon- 

 ditionally, for it was observed in young toad embryos that the postcardinals 

 resemble more nearly those of Urodeles (Hochstetter) where they surround the 

 pronephric ducts as vascular sheaths. It is true, the portion of the cylindrical 

 postcardinal network lying medially in Bufo is on the whole larger and doubtlessly 

 constitutes the main channel. But this channel and the portion of the postcardinal 



