556 
dorsal course, crosses above the shaft of the columella where, after 
giving off the chorda tympani, it passes into the proton of the di- 
gastric muscle. Dorsal and lateral to the nerve, the V. capitis lateralis 
and the A. facialis, also pass above the shaft. 
Third Stage of Development (fig. 8). 
Two embryos (total body lengths 12.5 and 13 mm.) are used as 
a basis for this description. One of them has been modelled. Changes 
-have been taking place rapidly, and the parts of the skeleton are, 
for the most part, well defined in cartilage. The strands of cells, 
which previously out- 
lined the base of the 
columella, have become 
membranous connec- 
tive tissue and binds 
the columella in the 
oval window. The colu- 
mella shows two centers 
of chondrification, one 
at either end. The 
chondrification from 
the proximal one has 
extended almost the 
entire length of the 
shaft, only a small 

Fig. 8. Embryo of 12.5 mm. total body length. . £ 
The general relations of the third stage of develop- Portion of it has chon- 
ment are shown here. b. boundary between columella drified from the other 
and extra-columella; f.o. fenestra ovalis; sg. squamosal ; H f th 
t.m. tympanic membrane; ¢.t. tympanic tube; £.(v.o.) center. Oreos e 
tympanum (ventral outpocketing). Other abbreviations first time do we find a 
Mia: line of division between 
columella proper and 
extra-columella. They are further differentiated by a more rapid 
chondrification of the former. The interhyale has chondrified from 
a center of its own. 
As was mentioned in the introduction, BENDER came to the 
conclusion that in Testudo graeca the extra-columella was a secondary 
element of the hyoid arch. In Chrysemys marginata the conditions 
seem to be different; the evidence for considering the extra-columella 
a primary part of the arch is strong. It certainly originates in the 
