DEVELOPMENT OF LYMPHATICS IN ANURA 97 
the confines of that channel. This is especially true during the 
stage when the lymph-heart anlage is plexiform, a section of 
which is shown in the photomicrograph, figure 16 (cor. lym. ant. 
sin.). As suggested in the figure, the masses of mesenchymal 
cells are not uniformly arranged around the outlines of the lymph- 
heart plexus, but are irregularly distributed, at one level being 
crowded against its lateral side, at another, against its medial, 
and more frequently between the meshes of its interanastomosing 
channels. Incidentally, it is evident that the reconstruction, 
which simply represents an enlarged cast of the lumen of the 
channels, does not exhibit all of the essential features of the 
developing structure, and to acquire a correct conception of the 
genetic processes, the sections as portrayed in the photomicro- 
graphs must be examined together with the reconstructions. 
The mesenchymal cells are so closely packed together and so 
filled with yolk globules that it is impossible to determine their 
individual boundaries. At several points, too, such aggregations 
seem to bound the cavity of the lymph heart anlage directly, 
at least no distinct intima lining it can be recognized. Otherwise 
the lining is quite sharply defined, though only part of the endo- 
thelial cells tend to the flattened shape, while others still retain 
the unspecialized form in which the nuclei are in general either 
oval or spherical and resemble those of ordinary mesenchymal 
cells. The nuclei of these mesenchymal masses stain deeply and 
are coarsely chromatic, and many of them have an indented 
circumference which conforms to the large yolk bodies in the 
cytoplasm. 
At another level of the lymph-heart plexus, Just back of the 
section shown in figure 16, primitive spherical blood cells, also 
stuffed with yolk globules, are crowded together and block the 
lumen of a connecting channel. Generally speaking, there is 
already a marked difference between the nuclei of circulating 
blood cells and those of mesenchymal cells; the former are dense 
and opaque and take almost a black color when stained with 
haemotoxylin, while the latter possess lighter staining areas 
between the large chromatic granules. A few exceptions, how- 
ever, were observed; several of the nuclei of the mesenchymal 
