NOTOCHORD AND VERTEBRAL COLUMN. 



551 



with each other by a delicate layer of tissue, and constitute the 

 substance in which the neural and haemal arches subsequently 

 become differentiated. 



At about the time when the first traces of the cartilaginous 

 sheath of the notochord arise, dif- 

 ferentiations take place in the 

 neural and haemal ridges. In the 

 neural ridge two sets of arches are 

 formed for each myotome, one 

 resting on the cartilaginous sheath 

 of the notochord in the region 

 which will afterwards form the cen- 

 trum of a vertebra, and constituting 

 a true neural arch ; and a second 

 separate from the cartilaginous 

 sheath, forming an intercalated 

 piece 1 . Both of them soon become 

 hyaline cartilage. 



There is a considerable portion 

 of the original tissue of the neural 

 ridge, especially in the immediate 

 neighbourhood of the notochord, 

 which is not employed in the for- 

 mation of the neural arches. This 

 tissue has a fibrous character and 

 becomes converted into the peri- 

 chondrium and other parts. 



The haemal arches are formed 

 from the haemal ridge in precisely 

 the same way as the neural arches, 

 but interhaemal intercalated pieces 

 are often present. In the region 

 of the tail the haemal arches are 

 continued into ventral processes 

 which meet below, enclosing the aorta and caudal veins. 



1 The presence of intercalated pieces in the neural arch system of Elasmobranchii, 

 Chimrera, etc. is probably not the indication of an highly differentiated type of 

 neural arch, but of a transitional type between an imperfect investment of the spinal 

 cord by isolated cartilaginous bars, and a complete system of neural arches like that 

 in the higher Vertebrata. 



FIG. 318. SECTION THROUGH 

 THE VERTEBRAL COLUMN OF AN 

 ADVANCED EMBRYO OF SCYI.LIUM 

 IN THE REGION OF THE TAIL. 



tia. neural arch ; ha. haemal 

 arch ; ch. notochord ; sh. inner 

 sheath of notochord ; ne. membrana 

 elastica externa. 



