DEVELOPMENT OF LYMPHATICS IN ANURA 99 
open communication with the pronephric venous sinus and no 
valve has yet been established at this lymphaticovenous junction. 
In the subsequent period of development the wall of the lymph 
heart changes very slowly in character. Instead of increasing 
in thickness, it becomes relatively thinner during the time be- 
tween 9- and 13- or 14-mm. embryos, due, in the first place, to 
the progressive flattening of the cells of the intima layer; secondly, 
to the attenuation of the covering cells, and, thirdly, to the loss 
of the large yolk corpuscles. In fact, during these stages, the 
second or covering layer does not form a complete investment of 
the endothelium, for there are bare spots (figs. 20 and 21) where 
endothelium constitutes the only line of demarcation between 
the cavity of the lymph heart and the surrounding mesenchymal 
reticulum. ‘The scantiness of the covering layer at this time is 
probably explained by the slow specialization of its cells and the 
more rapid expansion of the heart lumen with the resultant 
stretching of its lining cells. In 16-mm. embryos it again forms a 
continuous single cell-sheet (fig. 23), though it is still quite as 
thin as the endothelial layer, and is composed of slender spindle- 
shaped cells which show delicate striations. These cells, differ- 
entiated, as we have seen, from the mesenchymal cell aggrega- 
tions so conspicuous during the initial stages of the lymph heart, 
compose the anlage of its muscle coat. 
Knower claims there is evidence that the cells of the muscle 
coat are derived from the adjacent myotomes, but the writer is 
unable to furnish proof for the contention. In 5-mm. embryos 
a radical difference already obtains between the cells of the 
myotomes and those which surround the lymph-heart anlage—a 
distinction strikingly revealed in figure 16. Nevertheless, this 
fact does not discount the possibility that in much earlier stages 
the potential muscle cells of the lymph heart may proliferate 
from the myotomal elements; but, if this is found to be true, then 
it will be equally true that other mesenchymal cells of the same 
region have a similar source, provided the absence of any visible 
difference whatsoever between the cells of the mesenchyme and 
those of the lymph-heart anlage is any criterion of the similarity 
of origin. 
