QS OTTO F. KAMPMEIER 
cell aggregations approach the haemal nuclei in density,though 
the writer is unable to demonstrate decisively the transition and 
conversion of one into the other. This observation immediately 
calls to mind the researches of Miller (713) and Allen (’13) on the 
development of the thoracic duct in the chick and the caudal 
lymph heart in Polistotrema stouti, respectively, where it was 
discovered that some mesenchymal cells were converted into 
blood cells during the early genetic stages of these lymphatics. 
Consequently the question arises: Does the incipient anterior 
lymph heart in Anura also function transiently as a haemopoietic 
organ? Do some of the cells of the mesenchymal masses contig- 
uous to and between the channels of the plexiform anlage become 
differentiated into blood corpuscles? All my efforts to demon- 
strate this proved futile in the face of that prime obstacle, the 
abundance of yolk, which obscures and erases the more delicate 
tissue distinctions. 
In 6-mm. embryos not only has the periphery of the lymph- 
heart lumen become more definite than in the previous stage, 
but the surrounding masses of mesenchymal cells are becoming 
more evenly spread out over its outer surface (fig. 17). 
In the next older stage (7 mm.) the rearrangement of the cells 
composing the walls of the lymph heart is such that in general 
two layers may be distinguished (fig. 18), a lining or internal 
layer and a covering layer, but which, as yet, are not sharply 
delimited. This indistinctness is further emphasized by the fact 
that the intimal cells are not all flattened, as we should expect 
in this relatively advanced stage, but still retain their general- 
ized character. Indeed, one is not able to discern any striking 
difference between them and the other mesenchymal cells; as 
regards size, form, and appearance, the nuclei seem identical. 
The yolk globules have decreased in number in both layers, and 
for the first time one can get a glimpse of the shape of the cell 
body. Some of the cells of the outer or covering layer are 
becoming definitely fusiform, with their long axis directed 
parallel to the circumference of the heart cavity. A considerable 
number of blood cells are present in the latter (figs. 18 to 22), 
a fact of no special significance, however, for the heart is in broad 
