CH. LIX.] 



FORMATION OF LIMBS AND HEAD 



845 



ribs grow outwards and forwards in the thoracic region, and some of 

 them meet together in front, and enter into the formation of the 

 sternum or breast-bone. 



The Limbs. At first there are no limbs, and then they jut out as 

 buds from the sides of the body. Each consists of an epiblastic 

 covering and a core of mesoblast. The central part of the mesoblast 

 condenses and forms the cartilaginous rudiments of the bones which 

 afterwards become ossified, and it also forms .the ligaments which 

 connect the bones together. Buds from the muscle plates, opposite 

 the limbs, grow into them to form the muscles, and nerves from the 

 corresponding segments of the spinal cord enter the buds (fig. 639). 

 Blood-vessels connected with the vessels of the body also appear. 



FIG. 639. Diagram of a transverse section of embryo and amnion, showing extension of muscle plates, 

 rudimentary limbs, and membranous vertebral column. 1, Spinal cord with nerve-roots; 2, 

 membranous vertebral column, formed from fused scleratogenous segments of protovertebrse ; 3, 

 descending aorta ; 4, ccelom ; 5, amnion cavity ; 6, primitive intestine ; 7, muscle plate extending 

 into body wall ; 8, bud of muscle plate into limb ; 9, muscle plate. 



The Head. In the early stages the head is merely a rounded 

 projection developed in the head fold of the embryo. Its anterior part 

 is bent sharply downwards in front of the anterior end of the body in 

 which the pericardium has been formed, and the cleft between the 

 front of the head and the pericardium is the stomadosal space or 

 primitive mouth cavity (figs. 640, 641, 642). At this time there is 

 no neck, but from the posterior part of the head to the side of the 

 pericardium, a series of five visceral arches, with four intermediate 

 clefts, extend round the sides of the foregut. As the neck forms, the 

 visceral arches move forward with the head, and their lower ends meet 

 in the middle line of the neck in front of the anterior end of the body ; 

 thus it comes about that the stomadoeal space is now bounded 

 laterally and below by the first or mandibular arches, and above by the 



