826 DEVELOPMENT. [CH. LVIII. 



The Limbs. The muscles of the body developed from the 

 lateral extensions of the pro to vertebrae are, at first, like the 

 vertebrae, arranged in successive segments or myotomes. This is 

 very well seen in the ringed condition of the muscles in such 

 simple vertebrates as amphioxus. Even in fishes, where the limbs 

 are not in a high state of development, the muscular segments are 

 well seen. They are seen also in man in the intercostal muscles, 

 and in the abdominal region are indicated by the transverse septa 

 across the rectus abdominis, but here, as in other mammals, this 

 simple metameric segmentation is masked by the great develop- 

 ment of the large muscles which attach the limbs to the four 

 corners of the trunk. 



The limbs are lateral extensions of segments or somites in 

 certain situations. They consist of parietal mesoblast covered by 

 epiblast. At first there is simply a bud, but this grows, and in 

 time divides into three segments, arm, fore-arm, and hand in the 

 upper limb ; thigh, leg, and foot in the lower limb. The hand 

 and foot then give rise to buds corresponding to the digits. Each 

 limb is connected to a limb girdle. The epiblast here, as else- 

 where, forms the epidermis ; the true skin, subcutaneous tissues, 

 muscles, blood-vessels, and cartilages (subsequently replaced by 

 bone) are formed by differentiation from the mesoblast. In 

 further development the positions of the limbs become shifted by 

 rotation, so that the anterior (radial) border of the upper limb 

 becomes outermost, and the anterior (tibial) border of the lower 

 limb becomes internal. 



Formation of the Head. 



In the formation of the head, a number of elements are con- 

 cerned. There is first the notochord, which extends as far 

 forwards as the dorsum sellae ; this, however, as in the vertebral 

 column, is transitory, and is soon replaced by a primitive cartila- 

 ginous cranium developed from the mesoblast around it, as the 

 vertebrae are developed from the protovertebrae. This forms the 

 base of the skull. The roof or cranial vault is formed by mem- 

 brane bones, that is, bones not preceded by cartilage ; sense 

 capsules which are formed around prolongations of the brain, and 

 the visceral arches and slits contribute to the formation of that 

 part of the head which is called the face. The mesoblast, which 

 continues up the protovertebrae into the head region on each side 

 of the notochord, is not separated into parts corresponding to 

 vertebrae. Cartilage is formed in it ; two cartilaginous bars, one 

 on each side of the notochord, are called the parac/iordal cartilages, 



