816 



GENERATION. 



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 vertebral column, though 

 it is not itself developed into the vertebrae, which grow around it and encroach 

 upon its substance until it finally disappears. In many mammals the noto- 

 chord presents a slight enlargement at the cephalic extremity, which extends 

 to the auditory vesicles and it is somewhat diminished in size at the caudal ex- 

 tremity. By the sides of this cord are masses of cells which unite in front 

 of the neural canal and eventually are developed into the vertebrae. These are 



called protovertebrae, or somites, and 

 are shown in Fig. 303 (C, in A and B). 

 Twelve pairs of protovertebrae are 

 shown in Fig. 303, C. In the chick, 

 two pairs are first formed in the up- 

 per cervical region, on the second 

 day. They rapidly increase in num- 

 ber, 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 or 

 at the lowest part of the vertebral 

 column. The vertebrae, as they are 

 developed, are formed of temporary 

 cartilaginous structure, gradually ex- 

 tending around the chorda dorsalis, 



FIG. 301. The first six cervical vertebrae of the em- 

 bryon of a rabbit one inch in length (Robin). 



a, 6, cephalic portion of the notochord, exposed by 

 the removal of the cartilage ; b, portion of the 

 chorda dorsalis slightly enlarged, which, in this 

 embryon, 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 ; /. fc, en- 

 largements of the chorda dorsalis, between the 

 vertebrae ; g, cartilage of the lateral portion of 

 the atlas ; h. lateral portion of the axis ; i, t, 

 transverse apophyses of vertebrae. 



FIG. 302. Human embryon, about one month old, 

 showing the large size of the head and up- 

 per parts of the body, the twisted form of the 

 spinal column, the rudimentary condition of 

 the upper and lower extremities and the rudi- 

 mentary tail at the end of the spinal column 

 (Dalton). 



which then occupies the axis of the spinal column. These cartilages are not 

 divided at the lines of separation of the protovertebrae, but the protovertebrae 

 fuse together and the cartilages which are to be developed into the bodies of 

 the vertebrae are so divided off, that one cartilage occupies the place of the 

 adjacent halves of two protovertebrae. Between the bodies of the vertebrae, 

 the chorda dorsalis presents regular enlargements surrounded by a delicate 

 membrane. As ossification of the spinal column advances, that portion of 



