DEVELOPMENT OF LYMPHATICS IN ANURA 77 
be noted that the outer limit of the principal or mandibular 
plexus is sharply defined by a pair of broader, longitudinal vessels 
which genetically represent the oldest portion. In this stage (a 
7-mm. embryo) also the anlagen of the other divisions have made 
their appearance as outgrowths from the principal one. Anteri- 
orly, the circumoral division (s?. circor.) 1s an extension, on 
either side, growing forward along a ring-like vessel which is a 
branch of the external jugulars and encircles the mouth opening. 
Eventually the two halves of this division, by further elongation, 
meet and unite in front of it. Posteriorly, the pericardial di- 
vision (si. pericard.) of the sinus consists of a pair of caudally 
directed extensions, closely accompanying the external jugular 
veins towards the sinus venosus, where in subsequent stages they 
become prolonged outward to join the terminal portion of the 
temporal division of the same side. In the reconstruction under 
consideration each temporal division (s?. temp.) is shown as a 
derivative of a slender lymph vessel which extends laterally 
around the oropharyngeal cavity in association with a tributary 
of the external jugular vein. At this time the temporal division 
has already made considerable advance in plexus formation, but 
further tips continue to proliferate and to anastomose. One of 
these offshoots, passing back in the broad expanse of loose tissue 
lateral to the aortic arches, is highly distended locally (fig. 30), 
a condition manifestly produced by the pressure of the lymph 
collected within its lumen. A similar saccular enlargement of 
the temporal plexus exists also on the opposite side. In several 
sections on the right side, the writer was unable to follow with 
certainty its connection with the remaining part of the plexus, 
and on the reconstruction this point has been indicated by an 
vena juglaris externa dextra; the heavy masses in the center of the figure are mus- 
cle anlagen; in a curved line from lym. to lym., sections of the interanastomosing 
channels of the primary maxillary lymph sinus during its plexiform stage (cf. 
fig. 30). 
Fig. 12 Photomicrograph of a transverse section through the ventral ceph- 
alice region in an 8-mm. embryo of Bufo vulgaris (KX. E. C., series B 49, slide 1, 
section 88).- X 290. lym., the channels of the plexiform primary maxillary 
sinus are beginning to coalesce with one another by their expansion. Other 
references as in figure 11. 
