152 THE NEURAL AND H.EMAL ARCHES. 



from the adjoining tissue of the arches. Gegenbaur's figure 

 gives an excellent representation of the appearance of this 

 layer at the period under consideration. It is formed of a 

 homogeneous basis containing elongated concentrically arranged 

 nuclei, and constitutes a uniform unsegmented investment for 

 the notochord (vide PL Xil. fig. 10). 



The neural and hsemal arches now either cease altogether 

 to be united with each other by a layer of embryonic cartilage, 

 or else the layer uniting them is so delicate that it cannot be 

 recognised as true cartilage. They have moreover by stage P 

 undergone a series of important changes. The tissue of the 

 neural arches does not any longer form a continuous sheet, but 

 is divided into (1) a series of arches encircling the spinal cord, 

 and (2) a basal portion resting on the cartilaginous sheath of 

 the notochord. There are two arches to each muscle-plate, one 

 continuous with the basal portion of the arch-tissue and forming 

 the true arch, which springs opposite the centre of a vertebral 

 body, and the second not so continuous, which forms what is 

 usually known as the intercalated piece. Between every pair 

 of true arches the two roots of a single spinal nerve pass out. 

 The anterior root passes out in front of an intercalated piece 

 and the posterior behind it \ 



The basal portion of the arch-tissue likewise undergoes 

 differentiation into a vertebral part continuous with the true arch 

 and formed of hyaline cartilage, and an intervertebral segment 

 formed of a more fibrous tissue. 



The hsemal arches, like the neural arches, become divided 

 into a layer of tissue adjoining the cartilaginous sheath of 

 the notochord, and processes springing out from this opposite 

 the centres of the vertebrae. These processes throughout 

 the region of the trunk in front of the anus pass into the 

 space between the dorsal and ventral muscles, and are to be 

 regarded as rudiments of ribs. The tissue with which they are 

 continuous, which is exactly equivalent to the tissue from 

 which the neural arches originate, is not truly a part of the 

 rib. In the tail, behind the anus and kidneys, the cardinal 



1 In the adult Scyllium it is well known that the posterior root pierces the 

 intercalated cartilage and the anterior root the true neural arch. This however 

 does not seem to be the case in the embryo at stage P. 



