Charles R. Stockard 489 
Fig. 15. The stippled area of these figures does not necessarily indicate 
endoderm but merely the epithelial lining of the mouth and pharyngeal 
cavities. In some figures this, of course, is actually ectoderm. The 
outer body wall and brain sections are indicated in black. 
An embryo slightly older than the former, having almost lost its nasal 
communication with the throat and having one or two more gill clefts 
present, shows the mandibular cleft as follows: The arch of the younger 
embryo has almost entirely disappeared, the ventral mesenchymatous 
tissue, Ms, has thickened below the throat cavity and seems to lave 
pushed the middle curve of the arch up until in section it appears as a 
horizontal cleft extending from side to side of the head. In this con- 
dition the intimate fusion of the diverticula with the ectoderm still per- 
sists. A slight median furrow now runs along the floor of the throat 
throughout the region of the mandibular cleft. Fig. 2, a section through 
the widest portion of the mandibular region, illustrates these changes. 
Somewhat further caudad this arch also has its lateral borders directed 
slightly dorsal, as a reminiscence merely of the condition of the younger 
stage. The hyomandibular and other gill clefts of this embryo differ but 
slightly from those of the younger one. 
A somewhat more advanced embryo, measuring about 15 mm. in 
length, shows the mandibular cleft with its lateral portions curving 
down ventrally although still fusing with the ectoderm. Fig. 53 shows 
this condition in cross section; the embryo was shghtly shrunken so 
that the mouth cavity is exaggerated in the figure, yet the increase in 
the ventral mesenchyme and all points are well shown. The hypophysis, 
which is seen in Fig. 2 has terminated before reaching this corresponding 
region in the older embryo. The anterior part of the head has be- 
come much longer, a great nose region now extending in front of the 
forward end of the gut. This section is also far posterior of both the eyes 
and of the infundibulum while a similar part of the mandibular cleft 
in Fig. 2 was only one section posterior of the eyes and the infundibulum 
had not been reached, the fore-brain being shown bent under the mid- 
brain. 
Fig. 4 is a section through the mandibular cleft of an older embryo. 
In this stage all of the gills have appeared, but are still spread out on 
the lateral plates, and the nose exists at this time as two parallel tubes, 
a condition occurring also in the embryo of Fig. 3. The present section 
shows clearly the ventral inclination of the lateral diverticula of this 
cleft, as well as the ectoderm pockets, which appear almost ready to break 
through and form the lateral mandibular gill openings, a process, however, 
which does not take place: it is only at a later period that the definite 
