NEUKAL AECHES 



295 



ment of the arches is that which occurs in the hinder trunk region 

 of the Lamprey (Fig- 146). In this animal, as is well known, the 

 dorsal (sensory) and ventral (motor) nerve-roots are still separate 

 and are spaced out alternating with one another at approximately 

 equal distances along the sides of the spinal cord. The dorsal arch 

 elements alternate, in their turn, with the nerve-roots, so that there 

 are, on each side, an anterior (A) and a posterior (B) neural arch- 

 element within the limits of a single myotome. 1 It should be 

 noticed particularly that of these the anterior is situated between 

 the sensory and the motor nerve-root belonging to the segment. 

 This suggests a possible explanation of the later evolutionary history 

 of these cartilages (A) which in the typical Fishes tend very usually 



b. it it 



Fig. 147. — A, arrangement of arch-elements in mid-trunk region of a Carcliarias embryo 

 85"mm. in length ; B, do. in anterior region of a Sturgeon (Acipenser huso) 36 mm. 

 in length. (After Schauinsland, 1906.) 

 A, anterior neural arch-element ; B, posterior do. ; a, anterior haemal arch-element ; &, posterior 

 do. ; d.r, sensory nerve-root ; v, blood-vessel ; v.r, motor nerve-root. 



to become reduced in size, even to the point of disappearance. It 

 may be that this reduction in size is connected with the fact that 

 in the Fishes, as indeed in all gnathostomatous vertebrates, the two 

 nerve-roots have become approximated together to form a common 

 sensori-motor spinal nerve. On the other hand this explanation 

 would leave untouched the fact that a similar reduction in size may 

 occur in the corresponding ventral or haemal arches. 



The reduction in size of the "A" elements, which is of so 

 frequent occurrence amongst the typical fishes, is well shown in 

 Figs. 147, A and B, which are based upon Schauinsland's recon- 

 structions. This marked reduction is by no means of universal 

 occurrence. The two common Dog-fish — Scyllium and Acanthias — 

 are familiar examples of fishes in which the "A" elements 



1 In the anterior trunk region the arrangement is apt to be modified— the inter- 

 segmental vessel, which forms the anterior limit of the segment, coming to lie on the 

 tailward side of the A cartilage of that segment (Schauinsland). 



