DEVELOPMENT OF THE EMBRYO. 161 



The Internal Ear is developed from the auditory vesicle, budding 

 from the third cerebral vesicle; the membranous vestibule appears first, 

 and from it diverticula are given off, which become the semicircular 

 canals and cochlea. 



The cavity of the tympanum, the Eustachian tube, and the external 

 auditory canal are the remains of the first branchial cleft; the cavity of this 

 cleft being subdivided into the tympanum and external auditory meatus by 

 the membrana tympani. 



The Skeleton. The chorda dorsalis, the primitive part of the verte- 

 bral column, is a cartilaginous rod situated beneath the medullary groove. 

 It is a temporary structure, and disappears as the true bony vertebrae 

 develop. On either side are the quadrate masses of the mesoblast, the 

 primitive vertebrae, which send processes upward and around the medul- 

 lary groove, and downward and around the chorda dorsalis, forming in 

 these situations the arches and bodies of the future vertebrae. 



More externally the outer layer of the mesoblast and epiblast arch 

 downward and forward, forming the ventral laminae, in which develop 

 the muscles and bones of the abdominal walls. 



The true cranium is an anterior development of the vertebral column, 

 and consists of the occipital, parietal and frontal segments, which corre- 

 spond to the three cerebral vesicles. The base of the cranium consists, at 

 this period, of a cartilaginous rod on either side of the anterior extremity 

 of the chorda dorsalis, in which three centres of ossification appear, the 

 basi-occipital, the basi-sphenoidal, and the pre-sphenoidal. They ultimately 

 develop into the basilar process of the occipital bone and the body of the 

 sphenoid. 



The entire skeleton is at first either membranous or cartilaginous. At 

 the beginning of the second month centres of ossification appear in the 

 jaws and clavicle; as development advances, the ossific points in all the 

 future bones extend, until ossification is completed. 



The limbs develop from four little buds projecting from the sides of the 

 embryo, which, as they increase in length, separate into the thigh, leg and 

 foot, and the arm, forearm and hand ; the extremities of the limbs undergo 

 subdivision, to form the fingers and toes. 



Face and Visceral Arches. In the facial and cervical regions the 

 visceral laminae send up three processes, the visceral arches, separated by 

 clefts, the visceral clefts. 



The first, or the mandibular arches, unite in the median line to form 

 the lower jaw, and superiorly form the malleus. A process jutting from 

 its base grows forward, unites with the fronto-nasal process growing from 



