An introduction to the study of the comparative anatomy of animals . ult of segmentationis the same—viz. the formation of a blastula. The next step is the formation of a two-layered embryo,which is effected in Hydra by a process known as multipolarimmigration. During the growth of the blastula the planes ofdivision of the blastomeres were radial, but now several of thecells undergo tangential divisions and the innermost of theirproducts pass into the blastocoele. Other cells, again, slipbodily from their positions in the blastula wall and pass intothe blastocoele, and eventually the latter cav
An introduction to the study of the comparative anatomy of animals . ult of segmentationis the same—viz. the formation of a blastula. The next step is the formation of a two-layered embryo,which is effected in Hydra by a process known as multipolarimmigration. During the growth of the blastula the planes ofdivision of the blastomeres were radial, but now several of thecells undergo tangential divisions and the innermost of theirproducts pass into the blastocoele. Other cells, again, slipbodily from their positions in the blastula wall and pass intothe blastocoele, and eventually the latter cavity is completelyfilled up by cells which have been derived by these twomethods from the blastula wall. Immediately after this theoutermost layer of cells— those of the blastula wall—divide rapidly by radial divisions and form a definite layer ofcolumnar epithelium sharply marked off from the inner mass ofcells. (Fig. 50, D, ec.) This outer layer may now be called theectoderm, the inner mass the endoderm. Digitized by Microsoft® DEVELOPMENT OF HYDRA 239. Fig. 50. Development of Hydra. A, the ovum filled with deutoplasts ; ec,ectoderm of the parent Hydra; m£, mesoglosa ; en, endoderm ;jS, blastula stage, in which some cells of the blastula wall arepassing in towards the interior ; die, segmentation cavity orblastocosle. C, the blastoccele nearly filled up by immigrant ;, an embryo further developed with sk, an external spiny, and skz,an internal protective case ; ec, ectoderm ; i, inner solid mass ofcells. £, the embryo still enclosed in its double protective case,flattening itself out. J^, the embryo creeping out of its protectivecases ; lettering as in A and Z>. G, the empty case after escape ofthe embryo. {B, original; the rest after Brauer.) Digitized by Microsoft® 240 COMPARATIVE ANATOMY All this while the embryo remains attached to the parent,and has no external protection save the thin gelatinoid coatdescribed above. The ectoderm cells now secre
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