. The study of animal life. Zoology. H food-canal. Where there is no hollow ball of cells, but some other result of segmentation, the formation of a gastrula is not so obvious. Yet in most cases some analogous infolding is demonstrable. â In the hollow sac of cells there are already two layers. The outer, which is called the ectoderm or epiblast, forms in the adult the outer skin, the nervous system, and the most important parts of the sense - organs. The inner, which is called the endoderm or hypo- blast, forms the lining of the most import- ant part of the food- canal, and of such appendages
. The study of animal life. Zoology. H food-canal. Where there is no hollow ball of cells, but some other result of segmentation, the formation of a gastrula is not so obvious. Yet in most cases some analogous infolding is demonstrable. â In the hollow sac of cells there are already two layers. The outer, which is called the ectoderm or epiblast, forms in the adult the outer skin, the nervous system, and the most important parts of the sense - organs. The inner, which is called the endoderm or hypo- blast, forms the lining of the most import- ant part of the food- canal, and of such appendages as lungs, liver, and pancreas which are outgrowths from it. But in all animals above the Sponges and Coelenterates, a middle layer appears between the other two. From thisâthe mesoderm or mesoblastâ the muscles, the internal skeleton, the connective-tissue, etc., are formed. 9. Some Generalisations.â{a) The '^ ; To realise that almost every organism from the sponge to the highest begins its life as a fertilised egg-cell, and is built up by the division and arrangement, layering and fold- ing of cells, should not lessen, but should greatly enhance, the wonder with which we look upon life. If the end of this constantly repeated process of development be. Fig. 37.âThe formation of the two-layered gas- trula from the invagination of a hollow sphere of cells. (From the Evolution of Sex; after Haeckel.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. New York, C. Scribner's sons [printed at the Edinburgh press]
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1892
