THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 183 



column, extending through the bodies of the vertebrae and the intervertebral 

 disks. The cells are at first of an epithelial nature (Fig. 159), but those within 

 the vertebral bodies become vacuolated and broken up into irregular, multinu- 

 clear masses which then disappear. The cord is thus first interrupted in the 

 vertebrae, leaving only the segments within the intervertebral disks. Later these 

 segments also undergo degenerative changes, but persist as the so-called pulpy 

 nuclei. 



While the notochord is morphologically the forerunner of the axial skeleton, 

 and persists as a whole in Amphioxus, and in part in Fishes and Amphibia, in 

 the higher forms it is almost exclusively an embryonic structure with little or no 

 functional significance. It differs in origin from the true skeletal elements and 

 becomes involved with them only to disappear as they develop. 



Cleft perichordal 

 Spinal nerve rJJ^'fffiiJpffiwvaBr' sneat h 



Intersegmental 



artery 



Notochord 



Myotome 



Parts of two 

 adjacent sclerotomes 



FIG. 160. Five myotomes and sclerotomes from sagittal section of human embryo of 5 mm. Bardeen. 



Each sclerotome is differentiated into a looser cephalic part and a denser caudal part, the two 



being separated by a cleft (fissure of von Ebner). 



The Vertebrae. The changes which occur in the ventro-medial parts of the 

 primitive segments to form the sclerotomes have already been described. At 

 the same time it was stated that the vertebrae, with the other types of connective 

 tissue around them, were derived from the mesenchymal tissue of the sclerotomes 

 (p. 167; see also Fig. 142). The segmentally arranged masses forming the 

 sclerotomes are separated by looser tissue in which the intersegmental arteries 

 develop. The arteries mark the boundaries between the sclerotomes (Fig. 160). 

 About the third week of development the caudal part of each sclerotome con- 

 denses to form a more compact mass of tissue, and a little later becomes 

 separated from the cephalic part by a small cleft (Fig. 161). From the denser 

 caudal part a secondary mass of tissue grows medially and meets and fuses with 

 its fellow of the opposite side, thus enclosing the notochord. The medial mass 



