186 



TEXT-BOOK OF EMBRYOLOGY. 



Ossification begins in the sternum about the end of the fifth month of foetal 

 life. In each of the two cephalic segments a single center appears; caudal to the 

 second segment a series of paired centers appears, and later the centers of each 

 pair fuse into a single center (Fig. 169). The paired centers possibly represent 

 epiphyses of the ribs. Sometimes, however, the centers appear as a single 

 series, that is, with no indication of a paired character. The ossification 

 of the most cephalic segment, along with the episternal cartilages, produces the 

 manubrium sterni. Ossification of the following six or seven segments and their 

 union produce the corpus sterni. The bars formed from the most caudal ribs 

 (excluding the false ribs) form the xyphoid process. This process remains car- 



Olfactory organ 

 Hypophysis 

 Visual organ 



Prechordal plate 



Auditory organ 



Parachordal plate 



Notochord 



Nasal septum 



Olfactory organ 

 Hypophysis 

 Visual organ 



Prechordal plate 

 Auditory organ 



Basal (parachordal) 

 plate 



Notochord 



FIG. 170. FIG. 171. 



FIG. 170. Diagram of first stage in the development of the cartilaginous 



primordial cranium. Wiedersheim. 

 FIG. 171. Diagram of later stage of same. Wiedersheim. 



tilaginous for a long period, and may be single, perforated, or bifurcated, de- 

 pending upon the degree of fusion between the two primary bars. 



The Head Skeleton. Topographically the skeleton of the head appears 

 as the cephalic part of the axial skeleton. Structurally it is decidedly different, 

 for it is adapted to different conditions. The neural tube here becomes differ- 

 entiated into the brain with its many and dissimilar parts. In connection with 

 the brain the complicated sense organs (nose, eye and ear) arise. A part of 

 the alimentary tract and portions of the visceral arches are also inclosed 

 within the head. The head skeleton is specially modified to accommodate 

 these highly developed organs, and becomes extremely complicated. In 

 general the skeleton in any part of the body adapts itself to the other structures 

 and not the other structures to the skeleton. 



The anlage of the skull is a mass of embryonic connective tissue which sur- 



