THE LIMBS. 



163 



Bibs and Sternum. The ribs are formed by separate cartilaginous transfor- 

 mation in extensions of the protovertebral mesoblast between the muscle-plates. 

 According to some, they grow out from the cartilaginous vertebrae, but become separate 

 before ossification begins. Similar deposits are formed in connexion with the other 

 vertebrae (except the coccygeal in man), but they here become united by ossification 

 with and form parts of the vertebrae (see Osteology, Vol. II.). At their ventral 



Fig. 192. SECTIONS OP THE VERTEBRAL COLUMN OF A HUMAN PCETUS OF EIGHT WEEKS. 



(From Kolliker.) 



A, transverse longitudinal section of several vertebras. 1, 1, chorda dorsalis, its remains thicker 

 opposite the intervertebral discs ; 2, is placed on one of the bodies of the permanent vertebrae ; 3, on 

 one of the intervertebral discs. 



B, transverse horizontal section through a part of one dorsal vertebra. 1, remains of the chorda 

 dorsalis in the middle of the body ; 2, arch of the vertebra ; 3, head of a rib. 



Fig. 193. SAGITTAL SECTION OF A DORSAL INTERVERTEBRAL LIGAMENT OF AN ADVANCED 

 SHEEP'S EMBRYO. (Kolliker.) 



La, l.p, anterior and posterior ligaments ; Li, intervertebral ligament ; &, &', cartilaginous ends of 

 two vertebral bodies, iv, w' ; c, enlargement of notochord in the ligament ; c', c", enlargements in the 

 cartilaginous ends of the vertebrae. 



extremities the first seven (thoracic) cartilaginous ribs become united on either side 

 into a longitudinal cartilaginous plate, and this afterwards joins its fellcw of the 

 opposite side to form the sternum (manubrium and body). The xiphoid is of later 

 formation (Parker). This mode of development of the sternum explains many of 

 the malformations in the shape of fissures of the sternum of different gradation 

 which have been observed. 



The Limbs. The limbs arise as outgrowths from the lateral part of the 

 trunk in the thoracic and pelvic regions in the third day in the chick and in the 

 third and fourth week in the human embryo. They appear as flattened semilunar 

 thickenings of the parietal mesoblast covered by epi blast, budding out from a lateral 

 ridge which is seen in the early embryo near the line of cleavage of the mesoblast 

 and close to the outer margins of the muscle-plates, and several of which subsequently 

 send prolongations into each limb (*) ; they are therefore connected with several 

 mesoblastic somites, as is also indicated by their nerve supply. 



1 This is the case in elasisobranchs (see fig. 194, from Balfour), but, according to Paterson, 



