296 EMBRYOLOGY OF THE LOWER VERTEBRATES 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 Reptiles. 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,Gadow) 

 and a smaller apical (supradorsal) 

 portion. The spine may segment 

 into three superimposed rod -like 

 portions. 



HAEMAL ARCHES. In the 

 Cyclostomes typical haemal arches 

 are absent, although possibly 

 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 an in the case of the neural arches. This 

 seems to be clearly indicated by Callorhynclius (Fig. 149). It is ;ils<> 

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

 It-UK Hi " in each segment has undergone reduction in size exactly 

 rafl tin- case \\iih the corresponding neural element (A). A 

 similar condition is found in many Elasmobranohe, though not in all. 

 the "" " elements lu-in-j in some cases apparently completely nhsnit, 



1 The e.NHinination of one of these Dog-fishes brings out another point of gfiin.il 

 importance namely th;it tin .uvh-eloinent ;is it increases in lisa is apt to spread 

 round a nerve mot in it> neighbourhood. The result is that in tin- :u lull t In- n< rve 

 roots may pass out, nut l.i-i\\n-n tin- aivh .-lenient.-, Imt through them. The leMOB 

 to be leai lit from this is that the topo^r.iphieal relation of skeletal elements to m-r\e 

 trunks in not to Lit Jal.. n a ml.tlliMr .-\ id. -HIT ;is ID the primitive situation of siieh 

 , in> M . 



FIG. 148. Arrangement of arch-elements 

 in anterior caudal region of a Siredou 50 

 nun. in length. (After Schauinsland, 

 1906.) 



in Fiji- 147. 



