. Text-book of embryology. Embryology. 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&

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. Text-book of embryology. Embryology. 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&
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Image ID: RDHFG8
. Text-book of embryology. Embryology. 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 pr00eSS 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 l