DEYELOPMEXT OF THE CEAXIULL 731 



mities into a strip of cartilage on each side, and thus the matrix of the 

 sternum is at first cleft in two behind the pre-sterual portion. Sub- 

 sequent fusion of these two lateral strips unites them into one ; and the 

 transverse division of the bone only appears from the result of ossification 

 in successive distinct centres. This fact possesses an interest in con- 

 nection with the tendency of the meso-sternuui and xiphi-sternum to 

 divide and to produce various degrees of the malformation termed 

 fissura sterni. 



In the lumbar region there is reason also to look upon part of the 

 transverse processes as representing costal elements, but it is only in 

 cases of abnormal formation that they are found distinct from the rest 

 of the vertebra. (See the Descriptive x\natomy, Vol. I., p. 22.) 



The sacrum is peculiar in presenting, thi'ust in and compressed be- 

 tween its strictly vertebral elements and the iliac bone with which it 

 is united, several bony pieces which may be regarded as interposed 

 ribs. The ossification of two of these occurs as early as the fifth or 

 sixth month of fostal life. 



2. THE HEAD. 



The head of the embryo consists at first, as already stated, of the 

 cranial part alone, the face, nose, and mouth being absent. Below 

 the cranium, and extending as far forward as the point of junction of the 

 anterior with the middle encephalic vesicle, is situated the pharyngeal 

 portion of the primitive alimentary canal, closed in anteriorly by the 

 inflection of the blastodermic layers. It is at this place that subse- 

 quently the opening of the alimentary canal to the exterior takes place 

 in what constitutes ultimately the isthmus of the fauces ; and in front 

 of this the buccal cavity, not yet existing, is afterwards formed. 



In the progressive development of the head the principal changes by 

 which its fundamental parts come into shape may be enumerated shortly 

 as follows, viz., First, increase of deposit and textural differentiation of 

 the mesoblastic substance for the formation of the cranial walls in their 

 basilar, lateral, and upper portions ; second, the interpolation of the 

 sense-capsules as connected with the formation of the rudiments of the 

 nose, eye, and ear ; third, the development of the cerebral hemispheres 

 and other parts of the brain from the three primary encephalic vesicles; 

 fovrtli, the occurrence of the several cranial inflections ; and fifth, the 

 new formation of outgrowths for the development of the parts of the face. 



1. The Craiiiuiii. — The basal portion of the cranium consists 

 primarily of two fundamental parts. Of these the i^osterior is dis- 

 tinguished by the presence of the prolongation of the notochord within 

 it as far forward as the part of the skull which afterwards becomes the 

 pituitary fossa. This portion, which may be named occipito-sphenoid, 

 is originally formed by the undivided investing mass of Rathke, which 

 surrounds the anterior extremity of the notochord, and contains the 

 matrix of the future basi-occipital and basi-sphenoid cartilages. By its 

 later extension to the sides, it forms the matrix of the exoccipitals and 

 the periotic mass of cartilage which surrounds the primary auditory 

 vesicles. The main part extends forward below the posterior and middle 

 primary encephalic vesicles, ending at the pituitary fossa. 



The anterior portion of the basis cranii may be called sphcno-ethmoid, 

 as containing the matrix of the pre-sphenoid, and the ethmoid 



