304 EMBKYOLOGY OF THE LOWEK VEKTEBEATES oh. 



surface of the neural arch, from which however it is marked off 

 by a thin fenestrated layer of bone (6). It will be seen from the 

 diagram that the bases of the neural and haemal arches and the 

 base of the rib-bearer enclose a space through which runs the vertebral 

 artery (y.a). The haemal arch-element passes horizontally outwards 



beyond the base of the rib-bearer and the 

 rib itself forms merely a prolongation 

 of the haemal arch-element, becoming 

 segmented off from its proximal portion 

 (" transverse process ") by the de - 

 velopment of an intercalary zone of 

 fibrillar joint tissue. A little way out 

 from its base the rib grows out into a 

 projection which is directed dorsally 

 and ^towards the median plane. This 

 dorsal process is prolonged into a liga- 

 ment which is attached at its end to 

 a mass of bony tissue developed on 

 the outer side of the rib-bearer and 

 indicated in the diagram by the dia- 

 gonal shading. 



In the larva of Salamandra macu- 

 losa the condition is found which is 

 illustrated by Pig. 153, B. The most 

 important difference from the condition 

 seen in Necturus is that the basal 

 part of the haemal arch-element has 

 become greatly reduced, and is now 

 attached to the notochordal sheath 

 merely by a thin thread of bone. 

 The rib-bearer grows out from the 

 haemal arch-element as before but it 

 A.t„,„k,,H,.i .\.„ i;,,-,-,,: r;, isshorter and is more completely fused 



trunk vertebra of Salamandra maculosa With the neural arch. The dorsal 



larva; 0, trunk vertebra of Triton alpestris pr00 eSS of the rib has increased in 

 larva. &, i bone ; /i. a, haemal arch-element ; , ,i i n . , , 



strength and now extends right to the 

 dorsal end of the rib -bearer and is 

 firmly attached to it, so that the rib 

 has assumed an obvious double-headed 

 character with a practically equally 

 strong dorsal and ventral attachment through the substance of the 

 rib-bearer to the neural arch. 



Finally in the larva of Triton alpestris the condition is found 

 which is illustrated by Fig. 153, C. The original basal part of the 

 haemal arch-element which lay ventral to the vertebral artery has 

 disappeared, so far as cartilage is concerned, its place being taken 

 by a thin thread of bone. The rib-bearer is shorter and stouter 

 than in Salamandra and its fusion with the neural arch still more 



Fig. 153. — Illustrating the attachment 

 of rib to vertebra in the Urodela 

 according to Goeppert (1896). 



h.a*, haemal process ; N, notochord ; n.a. 

 neural arch; r, rib; r.&, rib- bearer; % 

 dorsal process of rib ; v.a, vertebral artery. 

 [The diagonally shaded portion of A re- 

 presents bone.] 



