aE 
DEVELOPMENT OF LYMPHATICS IN ANURA 0 
first a cleft or vacuole in the cytoplasm’ between the large pro- 
minent yolk globules (fig. 10, A), or if the anlage be larger, it 
may consist of a number of crevices which soon coalesce to pro- 
duce a more conspicuous cavity, (figs. 9. A, and 10). The lumen 
naturally expands with the growth of the anlage, but during 
several successive stages the confines of these lymphatics, like 
those of the haemal vessels, remain irregular and of varying 
thickness and appear gnarled, particularly in section (fig. 10), 
owing to the groups of large ovoid yolk globules which they con- 
tain and which do not entirely vanish until a relatively late 
period of sinus formation. 
From now on the development of the smus makes rapid prog- 
ress. The discrete lymphatic anlagen of the same side establish 
continuity with one another by end-to-end fusion and begin to 
send out endothelial extensions in a ventromedial direction. 
These sprouts actively proliferate, branch and rebranch, and 
freely anastomose with one another in such a way as to produce 
a plexus, the meshes of which all lie in the same plane. As the 
identical condition prevails on the opposite side of the head, the 
two plexuses approach each other, meet and combine in the 
midline and so create a broad intricate network (fig. 11, from 
lym. to lym.) extending in a curved plane from the vicinity of 
one external jugular to that of the other through the loose mes- 
enchyme between the thyroid and the epidermis on the ventral 
surface of the head. This network is the anlage of the principal 
or mandibular division of the primary maxillary sinus and is 
shown in the reconstruction in figure 30 (sz. mand.). In the 
drawing, the vascular channels are pictured in a flat plane, though 
in reality the most distal structures bend dorsolaterally. It may 
* The formation of the lumen, as indicated, raises the question, whether it is 
of intracellular or intercellular origin. The answer rests partly on our definitions 
of ‘cell’ and ‘syneytium.’ Are the spaces of mesenchyme to be considered as 
Sntercellular’ or ‘intracellular’? The originally solid lymphatic anlagen, de- 
scribed above, are probabry, like other mesenchymal tissue, syncytial in nature, 
and accordingly I would look upon the vacuole-like beginnings of their lumen as 
being intracellular in situation. Subsequently, with the expansion of the lym- 
phatic anlage into a definite vessel and the appearance of distinct cell boundaries 
in its endothelium, the lumen acquires its intercellular character. 
