454 NEURAL AND H^MAL A KG HEX. 



were derived from the segmented muscle-plates, and then became fused 

 into a continuous sheath around the notoehord and nervous axis ; till 

 finally they became in still higher forms differentiated into vertebrae and 

 their arches. 



During the stage represented in fig. 28 c/, and somewhat before 

 the cartilaginous sheath of the notoehord is formed, there appear four 

 special concentrations of the mesoblastic tissue adjoining the noto- 

 ehord, two of them dorsal (neural) and two of them ventral (haemal). 

 They are not segmented, and form four ridges, seated on the sides of 

 the notoehord. They are united 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 notoehord arise, differentiations 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 notoehord in the region which will afterwards form the centrum 

 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 notoehord, 

 which is not employed in the formation 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 lut'inal ridge in precisely 

 the same way as the neural arches, but inter has mal intercalated 

 pieces are often present. In the region of the tail the hasmal arches 

 are continued into ventral processes which meet below, enclosing the 

 aorta and caudal veins. 



Since primitively the postanal gut was placed between the aorta 

 and the caudal vein, the haemal arches potentially invest a caudal 

 section of the body cavity. In the trunk region they do not meet 

 ventrally, but give support to the ribs. The structures just described 

 are shewn in section in fig. 318, in which the neural (na) and haemal 

 (ha) arches are shewn resting upon the cartilaginous sheath of the 

 uotochord. 



While these changes are being effected in the arches the carti- 

 laginous sheath of the notoehord undergoes important differentiations. 

 In the vertebral regions opposite the origin of the neural and luvmal 

 arches (fig. 318) its outer part becomes hyaline cartilage, while the 

 inner parts adjoining the notoehord undergo a somewhat different 

 development, the notoehord in this part becomes at the same time 



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

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

 arch, but of a transitional type between an imperfect investment of tbe spinal cord by 

 isolated cartilaginous bars, and a complete system of neural arches like that in the 

 higher Vertebrnta. 



