THE NEURAL AND H/KMAL ARCHES. 373 



The layer of tissue which forms the vertebral bodies rapidly 

 increases in thickness, and very soon, at a somewhat earlier 

 period than represented in Gegenbaur's PI. XXII. fig. 4, a distinct 

 membrane (Kolliker's Membrana Elastica Externa) may easily 

 be recognized surrounding it and separating it from the adjoin- 

 ing 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 con- 

 stitutes a uniform unsegmented investment for the notochord 

 (vide PI. 13, fig. 10). 



The neural and haemal 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 

 recognized 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 (i) 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 1 . 



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 haemal 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 



figures alluded to. Gotte's very valuable researches with reference to the origin of 

 this layer in Amphibians tend to confirm the view advocated in the text. 



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

 calated cartilage and the anterior root the true neural arch. This however does not 

 seem to be the case in the embryo at stage F. 



