SKELETON, MUSCULAR SYSTEM AND SKIN 



819 



DEVELOPMENT OF THE SKELETON, MUSCULAR SYSTEM AND SKIN 



Notochord. One of the earliest structures observed in the develop- 

 ing embryo is the chorda dorsalis, or notochord. This is situated be- 

 neath the neural canal and extends the entire length of the body. It is 

 formed of a cord of simple cells and marks the situation of the ver- 

 tebral column, though it is not itself developed into the vertebrae, 

 which grow around it and encroach 

 on its substance until it finally dis- 

 appears. In many mammals, the 

 notochord presents a slight en- 

 largement at the cephalic extrem- 

 ity, which extends to the auditory 

 vesicles ; and it is somewhat dimin- 

 ished in size at the caudal ex- 

 tremity. By the sides of this cord 

 are masses of cells (mesoblastic 

 somites), which unite in front of 

 the neural canal and were formerly 

 thought to be developed into the 

 vertebrae. 1 These are the so-called 

 protovertebrae (or somites) and are 

 shown in Fig. 233 (C, in A and B). 

 Twelve pairs of somites are shown 



in Fig. 233, C. In the chick two 

 pairs are first formed in the upper 

 cervical region on the second day. 

 They rapidly increase in number 

 from above downward, until at the 

 fourth day there are twenty-one 

 or twenty-two pairs. They are not 

 formed in the region of the head 



Fig. 231. The first six cervical vertebra of the 

 embryo of a rabbit (Robin). 



a, b, cephalic portion of the notochord, exposed 

 by the removal of the cartilage ; b, portion of 

 the notochord slightly enlarged, which, in this 

 embryo, was situated between the atlas and 

 the occipital bone ; c, odontoid process ; d, base 

 of the odontoid process ; e, inferior, or second 

 part of the body of the axis ; f, k, enlarge- 

 ments of the notochord, between the vertebrae ; 

 g, cartilage of the lateral portion of the atlas; 

 h, lateral portion of the axis ; i, i, transverse 

 apophyses of vertebrae. 



or at the lowest part of the verte- 

 bral column. The vertebrae, as they are developed, are formed of 

 temporary cartilaginous structure, gradually extending around the noto- 

 chord, which then occupies the axis of the spinal column. These carti- 



1 The mesoblastic somites (protovertebrse) are sometimes called muscle-plates, or myotomes. 

 It is thought that cells from their inner walls are concerned in the development of the vertebrae 

 and that the other cells are developed, some into muscular structure and some into the true 

 skin ; but their destination is somewhat obscure. The outer cells developed into cutis are 

 called the skin-plates, or dermatomes. 



