. Text book of zoology. Zoology. 44 General Part. 1 ar ge^ and tte segmentation cavity is wanting, or is very small. In sucli cases the gastrula appears to be formed in a manner different from the foregoing: the epiblast cells, which originally form a cap upon the hypoblast, gradually grow round it, and the hypoblast itself often encloses no archenteron, but forms a compact mass. In the first and second types of gasti-ula formation described above, the blastula grows directly into the gastrula by a modification of some cells to form the hypoblast (Fig. 29, 4, s). In the blastula, the outer end
. Text book of zoology. Zoology. 44 General Part. 1 ar ge^ and tte segmentation cavity is wanting, or is very small. In sucli cases the gastrula appears to be formed in a manner different from the foregoing: the epiblast cells, which originally form a cap upon the hypoblast, gradually grow round it, and the hypoblast itself often encloses no archenteron, but forms a compact mass. In the first and second types of gasti-ula formation described above, the blastula grows directly into the gastrula by a modification of some cells to form the hypoblast (Fig. 29, 4, s). In the blastula, the outer end of these cells is the wider, but they gradually alter in shape until the inner ends ai'e wider than the others, which results simply from a different ari-angement of then- protoplasm (protoplasmic move- ments). This change in the shape of the cells must necessarily lead to a flattening of one side of the blastula and then to its invagination. Simultaneously an alteration occurs in the epiblast cells: these become shorter and wider, so that they can extend over a large ai-ea. Whilst the invagination remains, the original relations of hypoblast and epiblast are i-etained. The epibolic gastrula (gastrula by overgi-owth) described under (3), is probably formed in the same way. There is no need to assume that the epiblast cells get loose from the hypoblast and grow over it. It seems that the wide outer ends of the hypoblast cells (Pig. 33, 1) gradually become narrower, and the inner ends broader, whilst the epiblast cells at the same time spread out, so that, by the same process as in the invagination of the typical gasti-ula, they gradually enclose the hypoblast, without its being possible to speak of an active migration of cells, and without the contiguous surfaces of the epiblast and hypoblast cells being Fig. 32. Gastrula of a maiine â Gastropod (Natica). formed in the same way as the gastrula in Pig. 31, from which it differs in the possession of an archenteron.â A
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
