THE EMBRYO. 



1167 



cerebral vesicles, from which all the different parts of the encephalon are developed. 

 The primary cerebral vesicles at this time freely communicate with each other at 

 the points of constriction. 



The three cavities are lined by epiblast and covered by the same structure. 

 Between these two layers of epiblast a layer of mesoblast spreads over the whole 

 surface of the cerebral vesicles and forms the membranous cranium. From these 

 structures the cranium and its contents are developed. The external layer of the 

 epiblast forms the epidermis and hairs of the scalp. The mesoblastic layer forms 

 the true skin, the blood-vessels (all but their endothelial lining), muscles, connec- 

 tive tissue, bones of the skull, and membranes of the brain. The layer of epiblast 

 lining the vesicles forms the nervous substance of the encephalon, while the vesicles 

 themselves constitute the ventricles. 



ANTERIOR 

 COM MON 

 LIGAMENT 



-POSTERIOR COMMON 

 LIGAMENT. 



-Cartilaginous end 

 of vertebral body. 



Enlargement of noto- 

 ~ chord in disc. 

 Intervertebral disc. 



-Enlargement of noto- 

 chord in the carti- 

 laginous end. 



FIG. 711. Sagittal section through the intervertebral disk and adjacent parts of two vertebrae of an advanced 

 sheep's embryo. (Kolliker.) 



The cephalic end of the notochord terminates in a pointed extremity which 

 extends as far forward as the situation of the future basi-sphenoid, and is imbedded 

 in a mass of mesoblast, the "investing mass of Rathke." The posterior part of 

 this mass, which corresponds to the future basi-occipital, shows a subdivision into 

 four segments, the three roots of the hypoglossal nerve indicating their lines of 

 separation. Two cartilaginous bars, the parachordal cartilages, then become de- 

 veloped in this investing mass, and these surround the notochord, meeting first on 

 its ventral and next on its dorsal aspect to form the lasilar plate, the anterior 

 margin of which forms the future dorsum sellse. From this plate are developed the 

 basi-occipital and basi-sphenoid, and by lateral expansions from it the ex-occipitals 

 and the greater wings of the sphenoid. On either side of the parachordal cartilage 

 a cartilaginous capsule, the labyrinthine or periotic cartilage, surrounds the otic 

 vesicle, and from it the petrous and mastoid portions of the temporal bone are 

 developed. In front of the investing mass of Rathke two lateral bars are directed 

 forward, enclosing between them a space, which forms the pituitary fossa, in which 

 the pituitary body is eventually developed. These bars are named the prechordal 

 cartilages or trabeculce cranii, and extend as far forward as the anterior extremity 

 of the head, Avhere they coalesce with each other to form the ethmoid plate (Fig. 

 712). This encloses the olfactory pits forming the cartilaginous nasal capsule, 

 from which the ethmoid and turbinated bones are developed. A portion of the 

 ethmoid plate remains unossified and constitutes the cartilaginous part of the nasal 

 septum and the cartilages of the outer nose. From the trabeculae cranii the pre- 

 sphenoid is developed, and from this two lateral expansions extend to form its 



