596 



EMBRYOLOGY. 



permanently in the lower classes. As Amphioxus possesses a 

 membranous axial skeleton, so the Selachians and certain of the 

 Ganoids are representatives of the stage with cartilaginous vertebral 

 column. The third stage in the evolution of the axial skeleton is 

 more or less completely attained by all the higher Vertebrates. 



This, again, is a very instructive example of which the embryology 

 of the skeleton presents many others of the parallelism which exists 

 between the development of the individual and that of the race ; it 

 teaches how embryological and comparative-ana- 

 tomical investigations are mutually complementaL 

 In the detailed description of the conditions 

 which are observed in the development of the 

 cartilaginous and bony axial skeleton, I shall limit 

 myself to Man and Mammals, and since great 

 differences exist between the posterior region, 

 which encloses the spinal cord, and the anterior, 

 which envelops the vesicles of the brain, I shall 

 treat of them separately. 



Tig. 325. Longitu- 

 dinal [frontal] sec- 

 tion through the 

 thoracic region of 

 the vertebral 

 column of a human 

 embryo 8 weeks 

 old, after KOL- 



L1KER. 



v, Cartilaginous 

 body of vertebra; 

 li, intervertebral 

 ligament; ch 

 chorda. 



(a) Development of the Vertebral Column. 



The process of chondrification commences in 

 Man at the beginning of the second month. At 

 certain places in the tissue enveloping the chorda 

 the cells secrete between themselves a cartilaginous 

 matrix, and move farther apart, whereas at other 

 intervening and narrower tracts the character of 

 the tissue is not altered (fig. 325). In this manner 

 the skeletogenous layer is differentiated into nu- 

 merous vertebral bodies (v), which in longitudinal sections are the 

 more translucent, and into the intervertebral discs (ligamenta 

 intervertebralia) which separate them (li). 



The process of chondrification, as FEOKIEP has followed it in the case of the 

 embryo calf, proceeds as follows: there arise on both sides of the chorda 

 masses of cartilage which are united on the ventral side of it by a thinner 

 layer. Somewhat later the cartilaginous half-cylinder is closed on the dorsal 

 side also. 



Upon the appearance of a segmented vertebral column the 

 chorda loses its function of a supporting skeletal rod. From this 

 time forward it therefore suffers a gradual obliteration. The parts 

 enclosed in the bodies of the vertebrae are restricted in their growth r 



