184 



TEXT-BOOK OF EMBRYOLOGY. 



thus formed may be considered as the anlage of the body of a vertebra. Another 

 secondary mass also grows dorsally between the myotome and the spinal cord, 

 forming the anlage of the vertebral arch. A third mass grows ventro-laterally to 

 form the costal process (Figs. 162 and 163). The looser tissue of the cephalic 

 part of each sclerotome also sends an extension medially to surround the 

 notochord, and fills up the intervals between the succeeding denser (caudal) 

 parts. The looser part also forms a sort of membrane between the succeeding 

 vertebral arches. The tissue between the denser caudal part and the looser 

 cephalic part of each sclerotome is destined to give rise to an intervertebral 

 fibrocartilage. While the denser tissue forming the caudal part of each sclero- 



Myotome 



Notochord 



Cleft 



Intersegmental artery 



Perichordal sheath 

 Intervertebral disk 

 Interdiscal membrane 



Fig. 161. — Six myotomes and sclerotomes from sagittal section of human embryo of 6 mm. 

 Bardeen. Compare with Fig. 160. 



tome probably gives rise to the greater part of a vertebra, the looser tissue of the 

 cephalic part is also involved in the formation of the cartilaginous body, as will 

 be noted again in the following paragraph. The peculiar feature of the process 

 is that the denser caudal part of a sclerotome becomes associated with the looser 

 cephalic part of the next succeeding sclerotome, so that each vertebra is derived 

 from parts of two adjacent sclerotomes and not from a single sclerotome. This 

 naturally brings about an alternation of vertebra and myotomes (Fig. 161). 



So far the anlagen of the vertebrae are in the so-called blastemal stage. 



Following the blastemal stage and beginning in human embryos of about 15 

 mm., comes the cartilaginous stage in which the mesenchymal anlagen of the 

 vertebrae are converted into embryonic hyalin cartilage. In the body of each 

 vertebra a center of chondrification appears in the looser tissue of the caudal 



