A treatise on zoology . ly in the JurassicCestracibntidae {Hyhodus, p. 144), but also in the Cladoselachii(p. 185), Acanthodii (p. 189), and Pleuracanthodii (p. 180). The notochord secretes an outer thin elastica externa and aninner fibrous sheath, often very thick (Hasse [201], Klaatsch [265],Schauinsland [384]). Outside there is the mesoblastic skeletogenouslayer (Fig. 60). Neural arches develop in this layer, and alsohaemal arches, which in the trunk are represented by lateral basalprocesses (parapophyses). In some fish, Polypteridae and the VERTEBRA L COL UMN 99 Actinoptcrygii, the notocho
A treatise on zoology . ly in the JurassicCestracibntidae {Hyhodus, p. 144), but also in the Cladoselachii(p. 185), Acanthodii (p. 189), and Pleuracanthodii (p. 180). The notochord secretes an outer thin elastica externa and aninner fibrous sheath, often very thick (Hasse [201], Klaatsch [265],Schauinsland [384]). Outside there is the mesoblastic skeletogenouslayer (Fig. 60). Neural arches develop in this layer, and alsohaemal arches, which in the trunk are represented by lateral basalprocesses (parapophyses). In some fish, Polypteridae and the VERTEBRA L COL UMN 99 Actinoptcrygii, the notochordal sheaths remain unljroken through-out ontogeny, the vertebral centra being formed round them inter-segmentally. Generally the verteljral rings, or half-rings, developedin connection witli the arches constrict the notochord, so as even toobliterate it in the middle of each centrum (Fig. 94). In other fish, the Dipnoi, the mesoblastic cells of the skeleto-genous layer at the base of the arches pierce the thin elastica. f-. ^ 1^—?^^yfs- ^-v- ??V — Fio. IlO. Scyllium canlcula, L. Transverse section of the vertebral column of the tail of a youngembryo. A,the base of the neural arch in an older embryo ; B, of a later stage ; C, all , caudal artery; c, cartilage; , elastica interna or inner layer of the fibrous sheath; el,elastica externa ; , fibrous sheath which becomes invaded by mesoblastic cells ; , haeniaiarch ; , neural arch ; , nerve-cord ; nt, notochord ; , notochordal epithelium ; nu,nuclei of mesoblastic cells passing througli the broken elastica externa ; v, caudal vein. externa, and invade the fibrous sheath into which they migi-ate atfour jDoints in its circumference in each segment. Here they giverise to a small amount of cartilage ; without, however, forming dis-tinct centra (Fig. 204). But in the Elasmobranchs the invasion ismuch more thorough. Almost the whole thickness of the Avidesheath is formed of intrusive mesoblastic tissu
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