254 



SKELETON OF EXTREMITIES 



Skeleton of extremities. The skeletal core of the limb-buds is produced 

 by a condensation of the vascular mesenchyme in the axis of the developing 

 limbs, and at their bases. The skeletal blastema becomes still further condensed 

 in the situation of the future bones, and distally into a hand or foot plate. 

 This stage is named the blastema or prechondral stage. The cellular blastema 

 is then converted into cartilage by the appearance of centres of chondrification 

 in the precartilaginous blastema of the several skeletal parts. During this 

 chondrogenetic stage the limbs acquire the main features of the adult form. 

 The joints are at first represented by areas of mesenchymatous condensation, 

 which directly join the cartilages, but by the end of the second month the 

 joint-cavities have appeared (fig. 248, p. 196) and the surrounding mesenchyme is 

 condensed and thickened into the capsular ligaments. Intra-articular structures are 

 derived from the primary blastema. Centres of ossification next appear in the 



FIG. 308. DIAGBAMS OP THE CABTILAGINOUS CRANIUM. (Wiedersheim.) 



A, First stage. 



Ch, notochord ; Tr, trabeculse cranii ; P.ch, parachordal cartilages ; P, situation of pituitary body ; 

 N, E, 0, situations of olfactory, visual, and auditory organs. 



B, Second stage. 



B, basilar cartilage (investing mass of Rathke) ; S, nasal septum and ethmoidal cartilage; Eth, 

 Eth', prolongations of ethmoidal around olfactory organ, completing the nasal capsule ; Ol, foramina 

 for passage of olfactory nerve-fibres ; N, E, 0, Ch, Tr, as before. 



cartilages, and they closely correspond to the preceding centres of chondrification 

 (Bardeen.) 



Development of the skull. As in the trunk, so in the head, the noto- 

 chord is at first the only supporting structure. As we have seen in an earlier 

 section, it extends forwards to the flexure of the mid-brain, and then returns on 

 itself to end at the attachment of the buccopharyngeal membrane (fig. 307). 

 Cartilage begins to be laid down, during the second month, in the mesenchyme 

 on each side of this part of the notochord, and a basicranial plate is formed enclosing 

 the chorda and extending from the future foramen magnum to the stalk of the 

 pituitary body, where it ends in a plate or process which becomes the dorsum settee. 



It appears from the work of Jacoby, Levi, Robinson, and others that there are no definite 

 parachordal nor trabecular cartilages in the human embryo such as occur in lower forms 

 (fig. 308). Levi describes the appearance of a number of chondroblastic centres, which ultimately 

 fuse to form the continuous chondrocranium. The basicranial plate is chondrified in two sections, 



