118 



DEVEL OPMENT. 



of the scalp. The mesoblastic layer forms the true skin, the blood-vessels, mus- 

 cles, connective tissue, bones of the skull, and membranes of the brain. The 

 layer of epiblast lining the cavity forms the nervous substance of the encephalon, 

 while the cavity itself constitutes the ventricles. 



The upper end of the notochord terminates at its cephalic end in a pointed 

 extremity which extends as far forward as the situation of the body of the future 

 sphenoid bone, and is there imbedded in a mass of tissue, the " investing mass of 

 Rathke." This mass, derived from mesoblastic tissue, becomes cartilaginous, and 



FIG. 91. Vertical section of the head in early embryos of the rabbit. Magnified. (From Mihalkovics). A. 

 From an embryo of five millimetres long. B. From an embryo of six millimetres long. c. Vertical section of the 

 anterior end of the notochord and pituitary body, etc. from an embryo sixteen millimetres long. In A, the fau- 

 cial opening is still closed. In B, it is formed, c. Anterior cerebral vesicle, me. Mesocerebrum. mo. Medulla 

 oblongata. co. Epiblast. m. Wall of medullary canal, if. Infundibulum. am. Amnion. spe. Spheno-ethmoidal. 

 be. Central (dorsum sellse), and spo, spheno-occipital parts of the basis cranii. h. Heart. /. Anterior extremity 

 of primitive alimentary canal and opening (later) of the fauces, i. Cephalic portion of primitive intestine. 

 tha. Thalamus. p'. Closed opening or the involuted part of the pituitary body (py). ch. Notocnord. ph. Pharynx. 



from it is developed the basi-occipital and basi-sphenoid bones ; and by lateral 

 expansions from it the occipitals, the greater wings of the sphenoid, and the 

 periotic mass of cartilage surrounding the primary auditory vesicles. From the 

 front of the investing mass of Rathke, which corresponds in position to the future 

 dorsum sellae, two lateral bars are directed forward, enclosing a space which forms 

 the pituitary fossa, in which the pituitary body is eventually developed. These 

 bars are named the trabeculce cranii, and extend as far forward as the anterior 

 extremity of the head, where they coalesce with each other. From them the pre- 

 sphenoid and lateral masses of the ethmoid are developed ; and from their 

 coalescence a process is prolonged downward to form a portion of the framework 

 of the face hereafter to be described. From the pre-sphenoids, which are developed 

 from these trabeculae, a lateral expansion takes place, which forms the orbito- 

 sphenoid or lesser wings of the sphenoid, enclosing the optic foramen. 



The portions of the base of the skull above enumerated are formed from car- 

 tilage ; the remaining parts, comprising the vault of the skull, are of membran- 

 ous formation. 



The head at first consists simply of a cranial cavity, the face being subse- 

 quently developed in the manner now to be described by a series of arches with 

 clefts between them (Fig. 92). On the outer surface of what represents the 

 upper neck region of the embryo four linear vertical grooves make their appear- 

 ance on each side. Corresponding grooves are also formed in the Avail of the 

 intestine, the hypoblast of the pharynx being thus brought into contact with the 

 epiblast of the outer surface of the body along the lines of the grooves. These 

 grooves represent the branchial or visceral clefts, which become actual perfora- 

 tions in the lower vertebrates, and place the cavity of the pharynx in communica- 

 tion with the exterior. On either side of each groove a thickening of the meso- 

 derm occurs, so that five ridges are formed, the first of which is in front of the 



