THE CRANIUM. 



105 



concavity looking forwards in the upper limb and backwards in the lower limb. 

 The limbs also come to be folded ventrally against the body of the embryo. 



From the manner in which the flattened limb-bud grows out from the lateral 

 ridge, it is obvious that its surfaces must at first be dorsal and ventral. The 

 dorsal surface afterwards becomes extensor and the ventral flexor. The anterior 

 edge is respectively the radial and tibial ; the posterior, the ulnar and fibular. As 



Fig. 195. OUTLINES OF THE ANTERIOR EXTREMITIES 



OF HUMAN EMBRYOES AT DIFFERENT AGES. (After 



His.) 



A, at four weeks ; B, at five weeks ; C, at seven weeks ; 

 D, at nine or ten weeks. 



development proceeds, a half rotation occurs in 

 opposite directions in the two limbs, resulting 

 in the middle flexure (elbow, knee) being directed 

 forwards in the upper, backwards in the lower 

 limb. 



The bones of the limbs are laid down as 

 cartilages which appear as separate differentia- 

 tions of the more centrally placed mesoblast, 

 a portion of mesoblast remaining for a time undifferentiated opposite eacn synovial 

 articulation. Within this a cleft subsequently appears, and enlarges to form the 

 synovial cavity, the mesoblast which bounds the cleft developing eventually into 

 the synovial membrane and capsular ligaments of the joint. 



The cranium. In the head the notochord extends as far forwards as the 

 mid-brain. Here also it is invested by a continuous mass of mesoblast, which sends 

 lateral prolongations over the neural canal as in the trunk (membrana reuniens). 



Fig. 196. DIAGRAMS OF THE CARTILAGINOUS CRANIUM. (Wiedersheim.) 



A, First stage. 



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

 JV, , 0, situations of olfactory, visual and auditory organs. 



B, Second stage. 



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

 Eth', prolongations of ethmoidal around olfactory organ, completing the nasal capsule ; 01, foramina 

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



The main difference in development between the cranium and vertebral column 

 consists in the fact that no separate cartilaginous deposits to form vertebras occur 



