296 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



(" intercalary pieces," " interdorsals ") remain nearly as well developed 

 in the adult as the " B " elements. 1 



In Lung-fishes (here and there) and in Urodele amphibians the 

 "A" pieces can still be recognized (cf. Fig. 148); they have also 

 been observed in the embryos of various Eeptiles. In this case 

 they usually lose their individuality at an early period, becoming 

 completely merged in the definitive neural arch formed by the " B " 

 elements lying next to them on their headward side, but in some 

 cases, e.g. in the tail region of Lacerta, they have been found to 

 persist as discrete structures even in the adult, forming a vestigial 

 second neural arch behind the main arch. 



The neural elements become prolonged dorsally and meet so as 

 to form a complete neural arch and the apex of this becomes pro- 

 longed as an unpaired piece in 

 the mesial plane to form the 

 neural spine. The complete neural 

 arch formed in this way frequently 

 becomes segmented up into separ- 

 ate pieces of cartilage. The 

 arcualia in such cases become each 

 divided into a larger basal (basi- 

 dorsal — B, interdorsal — A,G-adow) 

 and a smaller apical (supradorsal) 

 portion. The spine may segment 

 into three superimposed rod -like 

 portions. 



Haemal Arches. — In the 



Pig. 148.— Arrangement of arch-elements Cyclostomes typical haemal arches 



in anterior caudal region of a Siredon 50 r. . ! ij.-l t, _. „;i,i» T 



(After Schauinsland, Me absent > although pOSSlbly 



vestiges of them are represented 



by a continuous ridge of cartilage 



occurring in the tail region of 



Petromyzon where the neural arches have also been reduced to a 



similar continuous ridge (Schneider). 



Of haemal arch elements there were apparently primitively two 

 pairs to a segment just as in the case of the neural arches. This 

 seems to be clearly indicated by Callorhynchus (Fig. 149). It is also 

 well shown in the young Sturgeon (Fig. 147, B) where the anterior 

 element (a) in each segment has undergone reduction in size exactly 

 as was the case with the corresponding neural element (A). A 

 similar condition is found in many Elasmobranchs, though not in all, 

 the " a " elements being in some cases apparently completely absent. 



1 The examination of one of these Dog-fishes brings out another point of general 

 importance namely that the arch-element as it increases in size is apt to spread 

 round a nerve-root in its neighbourhood. The result is that in the adult the nerve- 

 roots may pass out, not between the arch-elements, but through them. The lesson 

 to be learnt from this is that the topographical relation of skeletal elements to nerve- 

 trunks is not to be taken as infallible evidence as to the primitive situation of such 

 elements. 



mm. in length. 

 1906.) 



Reference letters as in Fig 



