744 



DEVELOPMENT OF TEE MUSCLES. 



In order also to complete the history of the formation of the limbs, 

 it is necessary to take account of the changes of attitude the anterior 

 and posterior respectively undergo, as compared with the primary em- 

 bryonic position. In this the elbow comes first to be turned outwards 

 and then directed backwards, bringing the flexor surface of the upper 

 arm forwards, while the position of the flexor surface of the fore-arm 

 and hand, though generally and naturally inwards, may, by supination, 

 be brought forwards, and by pronation backwards, the "latter being the 

 permanent position given to the manus in most animals. In the hinder 

 limb, again, the thigh is turned inwards, so that in the higher animals 

 the flexor surface looks backwards, and in all animals the lower 

 division of the limb is turned inwards and the sole of the foot down- 

 wards, so that the extensor surface and dorsum look forwards. (See vol. i., 

 p. 122.) 



4. DEVELOPMENT ^ OP THE MUSCLES. 



The muscles of the trunk derive their origin from the muscular 

 plates previously referred to as lieing separated by difierentiation of the 



Fig. 543. 



WJ? 





Fig. 543. — Section through the Lumbar IIkgion of an EiiBRYO-CnicK of Four 

 Days (from Foster and Balfour). 



nc, neural canal ; jir, posterior root and ganglion of a spinal nerve ; ar, anterior root ; 

 ■»«/>, muscle-plate; cfi, notochord ; WB, Wolffian ridge; AO, aorta; Vca, cardinal 

 vein ; Wd, Wolffian duct ; Wb, Wolffian body with glomeruli ; ge, germinal epitlielium ; 

 Md, depression forming the commencement of the Miillcrian duct ; d, alimentary canal ; 

 M, mesentery ; >S'0, somatopleure ; SP, splanchnopleure ; 1', blood-vessels ; P2h 

 pleuro-peritoneal space. 



