120 DEVELOPMENT. 



The lower ends of their cartilages unite together to form the thyroid cartilage of 

 the larynx. 



Between the mandibular and maxillary processes the buccal cavity or mouth is 

 formed ; this therefore owes its origin to the formation of the processes, and 

 consists of mesoblastic tissue lined by epiblast. As has been already stated (page 

 108), the cephalic end of the embryo becomes remarkably curved on itself, the 

 fore- and mid-brain bending downward over the anterior portion of the original 

 blastodermic membrane, which remains within the body of the embryo and from 

 which the fore-gut is to be developed. This fore-gut terminates as a blind 

 extremity beneath the head (Fig. 91, A,/). Another prominence forms on the 

 ventral surface of the fore-gut, which represents the rudimentary heart (Fig. 91, 

 A, A). Between these two prominences, caused by the projection of the fore-brain 

 and the heart, an involution of the epiblast takes place, gradually deepening 

 until it comes in contact with the upper part of the alimentary canal. This is 

 the stomodceum or mouth, which becomes bounded by prominences constituting 

 the maxillary and mandibular processes. It is at first quite distinct from the 

 upper part of the alimentary canal, which, as we shall hereafter see, is formed by 

 the inner or splanchno-pleural layer of the mesoblast and the hypoblast, the two 

 cavities being separated by all the layers of the blastodermic membrane. A com- 

 munication between the two is, hoAvever, gradually effected by the absorption of 

 these layers at the anterior extremity of the primitive alimentary cavity and the 

 hinder portion of the epiblastic involution from which the mouth is formed. 



The branchial grooves are at first fully exposed on the surface of the neck 

 region of the body, but later a fold of skin grows backward from the lower 

 border of each mandibular process. This fuses below with the side of the body 

 and completely conceals the grooves, which disappear, with the exception of the 

 first. Both the internal and external parts of this persist, the former giving 

 rise to the Eustachian tube and the tympanic cavity, Avhile the upper portion of 

 the latter forms the meatus auditorius. 



Development of the Nervous Centres and the Nerves. The medullary groove 

 above described (page 107) presents, about the third week, three dilatations at 

 its upper end, separated by two constrictions, and at its posterior part another 



dilatation, called the rhomboidal sinus. Soon after- 

 ward the groove become a closed canal (medullary 

 canal), exhibiting corresponding dilatations. This is 

 the rudiment of the cerebro-spinal axis. As the 

 ^rasfcjsv embryo grows, its cephalic part becomes more curved, 



jtffflgr ^HH!^ and the three dilatations at the anterior end of the 

 primitive cerebro-spinal axis become vesicles distinctly 

 separate from each other (Fig. 90). These are the 

 cerebral vesicles anterior, middle, and posterior. The 

 anterior cerebral vesicle (situated, at this period, quite 

 below the middle vesicle) is the rudiment of the third 

 . ventr i^ e an d f tne P ar ts surrounding it viz. the 

 a^H optic thalami and all the parts which form the floor of 

 FIG. 93. section of the me- the third ventricle. The middle vesicle represents the 

 aqueduct of Sylvius, with the corpora quadrigemina. 

 lp1theiium ent 3 al An a terior ^ray Tne posterior vesicle is developed into the fourth ven- 

 matter. 4. Posterior gray matter, tricle, and its walls form the pons Varolii, cerebellum, 



5. Anterior commissure. 6. Pos- , , , , -in c 0.1 f J.-L. 



terior portion of the canal, closed medulla oblongata, and parts in the floor ot the lourtn 



by the epithelium only. 7. An- ,,+ 1 

 terior column. 8. Lateral column, ventricle. 



tenor^tl 01 !! poTeriorU^ . At an early period in the development of this 



primitive brain a protrusion takes place from the 



anterior vesicle, which is at first simple, but soon becomes divided into two parts 

 by an antero-posterior fissure. These expand laterally, and the cerebral hemi- 

 spheres and corpora striata are developed from them. In the roof of the fore- 

 part of the posterior cerebral vesicle a thickening takes place, forming the rudi- 



