THE SKELETON 49 
sense organs and their investing capsules. In the embryo it 
is penetrated for some distance at its base by the forerunner of 
the backbone—the chorda dorsalis. For this reason it appears 
to be in a certain sense a prolongation of the axial skeleton of 
the trunk. The visceral or facial portion of the skull hes postero- 
ventrally to the cranial. It is closely connected with the pharyn- 
geal section of the alimentary canal, the lateral walls of which 
tau 
Fic. 31.—A, SLIGHTLY DIAGRAMMATIC MEDIAN LONGITUDINAL SECTION THROUGH THE 
HEAD AND ANTERIOR PORTION OF THE TRUNK OF A HUMAN EMBRYO, SEVENTEEN 
TO EIGHTEEN WEEKS OLD. (After W. His.) 
cp., brain; op., optic vesicle ; md., mandibular arch ; pe., pericardium ; cd., heart ; aw., 
auditory vesicle ; J-ZV, branchial clefts. 
B, Empryo TORPEDO, as seen by transmitted light. (After H. E. and F. Ziegler.) 
“eeeuachory, pit; Wy-, hyoid arch; V., trigeminal nerve; ~-cd., ventricle; VZ, 
VIL, facial and auditory nerves; LX, glosso-pharyngeal nerve. Other references 
as for A. 
are,in the embryo, perforated by “ gill-clefts” (J-IV, Fig. 31, 
A), so called because their presence points back to a time in which 
this part of the alimentary canal served not only for taking in 
food, but for respiration, as is still the case in the lower Verte- 
brates. That the system of skeletal arches, which alternate with 
these clefts has, in man, undergone considerable modification and 
reduction (cf. Fig. 105) will not appear strange, when the 
biological conditions are taken into account. The only point of 
E 
