. A manual of zoology. Zoology. 4ol CHORDATA plate which has arisen from a fusion of dermal ossifications, liecomes trans- ferred to a deeper position, and contriluites to the completion of the axial skeleton. From what was said aliove aliout the relations of dermal ai\d true teetli it is readily seen that the lining of the mouth cavity is a source of meml irane 1 'ones. In descrining the axial skeleton, the notochord comes first. Tliis has already keen mejitionetl in connexion «'iih lower CMiordates. It persists in the cyclostomes, kul from tlu'm up\\' it is gradually re].)laced \>y t
. A manual of zoology. Zoology. 4ol CHORDATA plate which has arisen from a fusion of dermal ossifications, liecomes trans- ferred to a deeper position, and contriluites to the completion of the axial skeleton. From what was said aliove aliout the relations of dermal ai\d true teetli it is readily seen that the lining of the mouth cavity is a source of meml irane 1 'ones. In descrining the axial skeleton, the notochord comes first. Tliis has already keen mejitionetl in connexion «'iih lower CMiordates. It persists in the cyclostomes, kul from tlu'm up\\' it is gradually re].)laced \>y the vertebne arising around it. It is of entodermal origin (^fig. o). arising as a longitudinal kand of the epithelium of the archenleron (I. (if), and, kecoming cut off, comes to lie in the long axis of the kodv ke- tween digestive tract and ner\"ous system (7/, III); here it forms a cyclindrical rod consisting of a connecti\'e tissue which, as ahwulv said, resembles plant tissues kecause of the vesicular nature of its cells (hg. 30). In transverse section (tig. 511) tire noto- chord is surrounded \>\ three la\'ers. internallv ky a fikrous noloclicrdal sltcalii, then a)i elastic layer (not alwa}'s present^ the dasliui cxlcnia. so called kecause an elastica interna sometimes occurs inside the nolochordal sheath: autl lastly a skeletogenous la}'er (^\S"). This last is a connective-tissue la\'er and is therefore connected with the other conneclixe tissues which surround muscles, nerxes, etc. It de- serves special mention kecauseinit thecartilages and kones arise from which the X'crteknr and skull are formed. Cells from it can penetrate tissue; 55 skeleiogenous Q^g notochordal sheath, converliui; it into tissue; L b, io\xer process 01 (skeletogenous tissue.) likrous cartilage, thus enakling it to participate in the formation of the verteknu. Since the notochord and its envelopes are elastic and give under the strain of the muscles, they are nnsegmenled. Tlie segmentat
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
