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 lies postero- 

 ventrally to the cranial. It is closely connected with the pharyn- 

 geal section of the alimentary canal, the lateral walls of which 



cp. - 



md.* 



IX 



FIG. 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 ; pc., pericardium ; cd., heart ; au. , 



auditory vesicle ; I-IV, branchial clefts. 



B, EMBRYO TORPEDO, as seen by transmitted light. (After H. E. and F. Ziegler.) 

 ol., olfactory pit ; hy., hyoid arch ; V., trigeminal nerve ; cd., ventricle ; VII, 

 VIII, facial and auditory nerves ; IX, glosso-pharyngeal nerve. Other references 

 as for A. 



are, in the embryo, perforated by "gill-clefts" (I-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 



