. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 66 ANIMAL PHYSIOLOGY. has been formed. This is rather to be considered as a type of one form of segmentation than as applicable to all, for even at this early stage differences are to be noted in the mode of segmentation which characterize effectually certain groups of animals; but in all there is segmentation, and that segmenta- tion is Fig. 69.âThe cleavage of a frog's egg (10 times enlarged). A, the parent-cell; B, the two first cle
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 66 ANIMAL PHYSIOLOGY. has been formed. This is rather to be considered as a type of one form of segmentation than as applicable to all, for even at this early stage differences are to be noted in the mode of segmentation which characterize effectually certain groups of animals; but in all there is segmentation, and that segmenta- tion is Fig. 69.âThe cleavage of a frog's egg (10 times enlarged). A, the parent-cell; B, the two first cleavage-cells ; C, 4 cells; D, 8 cells (4 animal and 4 vegetative); S, 12 cells (8 animal and 4 vegetative'); f\ 16 cells (8 animal and 8 vegetative); (?, S4 cells (16 animal and 8 vegetative); H, .32 cells; /, 48 cells ; K, 64 cells ; X, 96 cleavage-cells; M, 160 cleavage- cells (12B animal and 32 vegetative). (Haeckel.) Segmentation results in the formation of a multicellular aggregation â which, sooner or later, incloses a central cavity (segmentation cavity, hlastocele). Usually this cell aggrega- tion {Uasiula, Uastosphere) is reduced to a single layer of in- vesting cells. The Gastrula.âEnsuing on the changes just described are others, which result in the formation of the gastrula, a form of cell aggregation of great interest from its resemblance to the Hydra and similar forms, which constitute in themselves inde- pendent animals that never pass beyond that stage. The blas- tula becomes flattened at one pole, then depressed, the cells at. 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 Mills, Wesley, 1847-1915. New York, D. Appleton
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