HAEMAL ARCHES 



297 



In various Fishes, for example Laemargus and Amia (also in 

 some Amphibia, see Fig. 153, B), the haemal arch-element in the 

 trunk region segments into two pieces— one of which carries the 

 rib and may become shifted dorsally, while the other becomes 

 displaced in a ventral direction. The ventral pieces come to form 

 projections downwards from the centrum of the vertebra on each 

 side of the aorta and have been termed "aortic supports." They 

 may be termed haemal processes as they appear to be homologous 

 with the knob-like structures bearing this name which are to be 

 seen in the caudal region of e.g. Laemargus, projecting inwards from 

 the haemal arch into the tendinous septum which underlies the 

 caudal aorta. 



In the caudal region the haemal arch-elements are commonly 



They bend round to meet one 



much longer than in the trunk, 

 another and are prolonged into 

 a haemal spine. These features 

 are associated with the extension 

 of the body in a dorsal and ventral 

 direction correlated with the use 

 of this region of the body for the 

 purposes of movement. 



Towards the head end of the 

 series it not uncommonly happens 

 in Cartilaginous fishes that the 

 haemal arch - element becomes 

 broadened out over the surface 

 of the notochord indicating the 

 beginnings of the evolution of 

 perichordal centra (see below). 

 This is well shown by Callorhyn- 

 chus (Fig. 149) where incipient 

 centra are distinctly seen, formed 

 by the much-enlarged and fused haemal arch-elements (a and b). 



In the air-breathing vertebrates there are no longer double sets 

 of complete haemal arch - elements but a distinct trace of this 

 condition is seen in such a Urodele Amphibian as Siredon (see 

 Fig. 148) where a large perforation through the haemal arch 

 element, traversed by the intersegmental blood-vessel, betokens its 

 double origin. 



A characteristic feature of the Amniota is that the haemal arch 

 (the cartilaginous forerunner of the " chevron - bone ") tends to 

 become displaced forwards so as to assume an intervertebral position 

 or even to become fused with the vertebra lying in front (Anguis — 

 Goette). 



Vertebral Centra. — Except in the case of Cyclostomes, Holo- 

 cephali and Lung-fishes, the elastic notochord becomes replaced 

 physiologically during development by the series of vertebral centra. 

 In the various subdivisions of the Vertebrata we find two distinct 



Piq. 149. — Portion of vertebral column of a 

 10 cm. embryo of Callorhynchus a few seg- 

 ments posterior to the hind end of the 

 skull. (After Schauinsland, 1906.) 



Reference letters as in Fig. 147. 



