954 OF GENERATION EMBRYONIC DEVELOPMENT. 



according to the investigations of Mr. Parker, 1 to be as follows. Beginning 

 with the scapula, the thin epiphysial ossification running along the vertebral 

 border is the supra-scapular bone, well shown as segmented from the scapula 

 in the ray and sturgeon, and large, but not quite segmented off" from the 

 scapula proper in the frogs. The whole intraspinous portion of the bone 

 represents the scapula proper forming the greater portion of the scapula of 

 the bird, but best differentiated as a distinct bone in the iguana and turtle. 

 The acromion process, with the whole of the spinous process, constitutes the 

 mesoscapula, which is seen partly segmented from the body of the scapula 

 in the pangolins, but is well shown also in the iguana. The neck of the 

 bone and the articular facet forming the gleuoid region have a separate in- 

 ternal ossification in the frog and toad. The coracoid process forms a large 

 separate bone in the ovipara, and in monotremes, of which only the part 

 corresponding to the head is developed in the human subject, which speedily 

 coalesces with the other elements forming the scapula. The portion of bone 

 around the coracoid notch, Avith the forepart of the supraspinous fossa, is 

 the prescapula. It is well seen as a distinct bar above the glenoid cavity in 

 certain lizards, and exists as a separate cartilage, even in the cat. The car- 

 tilaginous extremities of the clavicle are remnants of a primary rod of car- 

 tilage developed independently of the clavicles. The outer or mesoscapular 

 segment becomes attached to the acromion by fibrous tissue, and an imper- 

 fect synovial articulation. The inner segment divides into two, of which 

 the outer portion becomes attached to the true clavicle, whilst the internal 

 portion, corresponding to the omo-sternum (or so-called episternum) of frogs, 

 becomes converted into fibro-cartilage. The clavicle itself is a parosteal bar 

 which has grafted itself on a delicate rod of cartilage. 2 



793. In order to facilitate the comprehension of terms now very generally 

 employed both in Human and Comparative Anatomy in regard to the parts 

 of a vertebra, the following description is here appended. The complete 

 typical vertebra (Fig. 356, A) essentially consists, according to Prof. Owen, 3 

 of the centrum, around which are arranged four arches inclosed by processes 

 in connection with it, viz., superiorly, the neural arch, which incloses the 

 neural axis, and is formed by a pair of " neurapophyses" (n, n) and a neural 

 spine (ns} ; inferiorly the hcemal arch, which is in like special relation with 

 the centres of the circulation, but may be expanded around the Visceral 

 cavity generally, and which is formed of a pair of " haemapophyses " (/;, A) 

 and the " haemal spine" (hs) ; and two lateral arches, inclosing vascular canals, 

 which are bounded by the "diapophyses" (d, d) and the " parapophyses " 

 (p,p), and are completed by the " pleurapophyses " (pi, pi). Of thes'e ele- 

 ments, the centrum is the most constant ; and next to these are the neural 

 arches, which we find in every part of the vertebral column through which 

 the neural axis passes, and which are enormously developed in the cranial 

 segments, in accordance with the high development of their nervous mass. 

 The haemal arches are often almost entirely deficient, as in the cervical and 

 lumbar vertebrae of Man and the Mammalia ; but in the dorsal vertebrae 

 they are very largely developed, and the elements of the lateral arches are 



1 See his important and extensive series of observations in the Monograph on the 

 Structure and Development of the Shoulder Girdle and Sternum in the Vertebnita, 

 18(18, published by the Kay Society. 



Mr. Parker differentiates three kinds of ossification ; " endostosis," occurring in 

 the substance of hyaline ciirlihige ; " ectostosis," when the bony deposit occurs in the 

 almost structureless inner layrr of the periosteum ; and " parostosis,'' when the bone 

 is formed in the skin, subcutaneous fibrous mesh, or in aponeuroses. 



3 See his Archetype Skeleton, his Lectures on Comparative Anatomy, and his Dis- 

 course on the Nature of Limbs. 



