1168 



EMBRYOLOGY. 



lesser wings ; each of these arises by two roots, one above and one below the optic 

 nerve, and, uniting outside the nerve, enclose the optic foramen. The base of the 

 primitive cranium therefore consists of two parts, prechordal and parachordal : the 

 former receives the organ of smell and is indented by the eyeball ; the latter sur- 

 rounds the auditory vesicle. Thus it will be seen that the bones which form the 



Situation of olfactory pit. 



Pituitary fossa.. 



Trabecula_.l._ 

 cranii. 



Situation of 



otic vesicle. 



Parachordal- 



cartilage. 



Notochord.- 



Ethmoid cartilage 

 and nasal septum. 



Olfactory organ. 



^Extension around 

 olfactory organ. 



^Foramen for 



olfactory nerves. 



Eyeball. 

 Pituitary fossa. 



Investing mass 

 -of Rathke. 

 Otic vesicle. 



-Notochord. 



FIG. 712. Diagrams of the cartilaginous cranium. (Wiedersheim.) 



base of the skull are preceded by masses of cartilage, which together form the 

 chondrocranium. Those of the vault of the skull, on the other hand, are of mem- 

 branous formation, and are termed dermal or covering bones. They are developed 

 in the mesoblast which lies superficial to the primordial cranium, or in that which 

 lies subjacent to the epithelial lining of the foregut. They comprise the upper 

 portion of the tabular part of the occipital (interparietal), the squamous-temporals 

 and tympanic rings, the two parietals, the frontal, the vomer, the internal ptery- 

 goid plates, and the bones of the face. Some of them remain distinct through- 

 out life (e. g., parietal and frontal), while others join .with the bones of the 

 chondrocranium (e. g., interparietal, squamous-temporal, and internal pterygoid 

 plates). 



The head at first consists simply of a cranial cavity, the face and neck being 

 subsequently developed in the manner now to be described. 



In all vertebrate animals there is at one period of their development a series 

 of grooves in the upper neck region of the embryo. These are named the branchial 

 or visceral clefts, and in man are four in number from before backward. They 

 take origin as paired grooves or pouches from the side of the pharynx, and over 

 each groove a corresponding indentation of the epiblast occurs, so that the latter 

 comes into contact with the hypoblast lining the pharynx, and these two layers 

 unite to form thin septa, along the bottom of the grooves, between the pharyngeal 

 cavity and the exterior. In gill-bearing animals these septa disappear and the 

 grooves become complete clefts, the gill clefts, opening from the pharynx on to the 

 exterior ; perforation does not, however, occur in birds and mammals. In front 

 and behind each cleft the mesoblast becomes thickened in the form of arches, the 

 branchial arches (Figs. 713, 750). In the human embryo there are five pairs of 

 these arches, one in front of the first cleft, one behind the last, and the three 

 remaining ones between the first and second, the second and third, and the third 

 and fourth clefts, respectively. The first arch is named the mandibular ; the 

 second the hyoid ; the third the thyro-hyoid, while the fourth and fifth have no 

 distinctive names. In each arch there is developed a cartilaginous bar which gives 





