The principles of biology . by seeing a somite of thebody transformed into a head. If, then^ a vertebrate animalhad a segmental composition of kindred nature, we ought tofind it most clearly marked in the lowest Vertebrata, andmost disguised in the highest Vertebrata. But here, as be-fore, the fact is just the reverse. Among the Vertebrata ofdeveloped type, such segmentation as really exists remainsconspicuous—^is but little obscured even ia parts of the spiaalcolumn formed out of integrated vertebrae. Whereas in theundeveloped vertebrate type, segmentation is scarcely at alltraceable. The Amp
The principles of biology . by seeing a somite of thebody transformed into a head. If, then^ a vertebrate animalhad a segmental composition of kindred nature, we ought tofind it most clearly marked in the lowest Vertebrata, andmost disguised in the highest Vertebrata. But here, as be-fore, the fact is just the reverse. Among the Vertebrata ofdeveloped type, such segmentation as really exists remainsconspicuous—^is but little obscured even ia parts of the spiaalcolumn formed out of integrated vertebrae. Whereas in theundeveloped vertebrate type, segmentation is scarcely at alltraceable. The Amphioxm, Fig. 191, is not only withoutossified vertebrae; not only is it without cartilaginous re-presentatives of them; but it is even without anything likedistinct membranous divisions. The spiaal column existsas a continuous notochord: the only signs of incipient seg- THE MOKPHOLOGICAL COMPOSITION OF ANIMALS. 109 mentation, being given by its membranous sbeath, in theupper part of which quadrate masses of somewhat denser. tissue seem faintly to represent neural spines. Moreover,throughout sundry groups of fishes and amphibians, thesegmentation remains very imperfect: only certain peri-pheral appendages of the vertebras becoming defined andsoKdified, while in place of the bodies of the vertebrae therestill continues the undivided notochord. Thus, instead ofbeing morphologically composed of vertebral segments, thevertebrate animal in its primitive form is entirely withoutvertebral segments; and vertebral segments begin to appearonly as we advance towards developed forms. Once more, evidence equally adverse to the current hypothesismeets us on observing that the differences between the partssupposed to be homologous, are as great at first as at the vertebrate animal primordiaUy consist of homo-logous segments from snout to tail; then the segments saidto compose the skull ought, in the lowest Vertebrata, to showthemselves much more like the remaining segments thanthey d
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectbiology, bookyear1864
