OSSIFICATION OF THE VERTEBRA. 



13 



organs : d, the infundibulum ; e, the pineal gland ; c, protoverte- 

 brae ; /., notochord ; 1, 2, 3, 4, 5, visceral arches; V, Vll VIII, the 

 tri"-eir.inal, portio dura, and ninth and tenth pairs of cranial nerves ; *, 

 the' nasal process ; I the maxillary process ; .r. the first visceral cleft. 

 A, B, upper and under vievts of the head of a Chick at the end of the 

 second day. C, side-view at the third day. D, side-view at seventy- 

 five hours. E, side-view of the head of a Chick at the fifth day, 

 which has been subjected to slight pressure. F, head of a Chick at 

 the si.xth day, viewed from below. 



abdominal part of the ventral tube, to give rise to the trans- 

 verse processes and rihs. In fishes, the latter remain distinct 

 and separate from one another, at their distal ends ; btit, m 

 most reptiles, in birds, and in mammals, the ends of some 

 of the anterior ribs, on both sides, unite together, and then 

 the united parts coalesce in the middle line to f onn a median 

 subthoracic cartilage — the sternum. 



When ossification sets in, the centra of the vertebrae are 

 usually ossified, in great measure, from ringlike deposits 

 which closely invest the notochord; the arches, from two 

 lateral deposits, which may extend more or less into the 

 centrum. The vertebral and the sternal portions of a rib 

 may each have a separate ossific centre, and become distinct 

 bones ; or the sternal parts may remain always cartilagin- 

 ous. The sternum itself is variously ossified. 



Between the completely ossified condition of the verte- 

 bral column and its earliest state, there are a multitude of 

 gradations, most of which are more or less completely 

 reahsed in the adult condition of certain vertebrated animals. 

 The vertebral colimin may be represented by nothing but 

 a notochord with a structureless, or more or less fibrous, 

 or cartHagiuous sheath, with or without rudiments of carti- 

 laginous arches a7id ribs. Or there may be bony rings, or en- 

 sheathing ossifications, in its walls ; or it may have ossified 

 neural arches and ribs only, withoitt cartilaginous or osseous 

 centra. The vertebrae may be completely ossified, with very 

 deeply biconcave bodies, the notochord remaining persistent 

 in the doubly conical intervertebral sitbstance ; or, ossifica- 

 tion may extend, so as to render the centrum concave on 

 one surface and convex on the other, or even convex at each 

 end. 



