ORIGIN OF LYMPHATICS IN BUFO 171 
between the brilliant brownish yellow color of the yolk-rich 
tissue and the blue of the yolk-poor is very striking, and in the 
colored sketches an attempt has been made to express the dis- 
tinction as strongly. 
In succeeding stages, 6 and 7 mm. embryos, the anlagen of 
the lymph sinus have in the main received a lumen. Their 
confines are still thick and heavy and the contour of the cavity 
is irregular on account of the many large protruding yolk spheres 
packed in the cytoplasm of the endothelium. Frequently 
protoplasmic bridges or trabeculae traverse the cavity from side 
to side and nuclei have been observed to be included in them. 
Such a condition, coupled with the fact that yolk globules oc- 
casionally lie freely in the lumen of an anlage, would point to 
the derivation of that lumen from intracellular vacuoles by their 
enlargement and coalescence. By this time all of the initial 
lymphatics have grown much in length and have also expanded 
proportionately in caliber. Some of them still retain their 
original junction with the intima of the vein; others have lost 
this connection and exist in the mesenchyme as longer or shorter 
discrete endothelial and yolk-lined fusiform spaces. Other such 
spaces cling to the parent vein almost throughout their entire 
extent and hence bear an extraordinary resemblance to the extra- 
venous spaces of mammalian embryos. Blood corpuscles have 
not been seen in the cavities of these structures; yet this finding 
is not as significant as it might seem, for as the sketches show, 
even in the external jugular veins of the specimens studied there 
is a paucity of them as a result apparently of fixation currents 
and contractions. 
The transformation of the discontinuous bilateral lymphatic 
rudiments into one broad continuous sinus is a very rapid one. 
During the stages previous to older 7 mm. embryos the anlagen 
had developed chiefly in a longitudinal direction, that is, along 
the cranial portion of the external jugulars, but now they begin 
to germinate collateral sprouts in an oblique direction towards 
the mid-ventral line, especially at the level of the thyroid anlage. 
These sprouts again proliferate buds, the tips of which grow out, 
soon collide and fuse with one another, and their cavities become 
