DEVELOPMENT OF LYMPHATICS IN ANURA 87 
In 1891 Field published his excellent work on the development 
of the pronephros in the frog, and in a footnote (91, p. 240) 
described briefly, though correctly, a ‘peculiar sac’ which he 
found in 8-mm. embryos lateral to the myotomes at the niveau 
of the third nephrostome and joined to the postcardinal vein. 
‘Respecting the fate and the significance of this singular struc- 
ture,” he says, ‘“‘I have no suggestions to offer.” Reference to 
this observation is made by Gaupp (’99, pt. 2, p. 380) who ealls 
it a ‘Blutblischen’ of unknown function. We now know that 
this enigmatical organ is the anterior lymph heart. 
Hoyer (05), in his work on the formation of the lymphatic 
system in frog larvae, states that the anterior lymph heart makes 
its appearance during the stage when the external gills begin to 
vanish, as a small spindle-shaped evagination from the short 
anterior vertebral vein anlage at the point where this vein 
branches dorsally from the pronephric venous plexus. At that 
time the walls of the fusiform heart are composed of an inner 
endothelium and an outer layer of stellate cells. In 6-mm. 
embryos the heart has become larger, but it is still in broad, open 
communication with the vein and contains numerous blood cor- 
puscles in its cavity. Its walls become thicker and a few cross- 
striated muscle fibers are visible in the outer coat. During this 
stage in the living specimen the heart occasionally quivers, but 
distinct rhythmic contractions do not become evident until later, 
when the embryo has reached the length of 12 mm. and the muscle 
elements have increased in number and in configuration. In the 
meantime valves have appeared, one at the junction of heart and 
vein and another at the opening of the lymph vessel into the 
heart. After the formation of these structures, blood cells are 
only exceptionally found in the heart chamber. The funda- 
mental changes in the development of the heart have now occur- 
red, and in subsequent stages it merely grows larger and acquires 
its definitive character. But it retains its original position 
lateral to the second myotome throughout the entire period of 
genesis and growth up to metamorphosis. 
Knower, in a short paper (08), remarks that the anterior 
lymph heart is the first lymphatic to be formed in the frog and 
