An introduction to the study of the comparative anatomy of animals . , Forty-eight-cell stage, showing the smaller cells at the upper pole andlarger yoke-laden cells at the lower pole. £!, F, two sides of the same ovumin later stages of segmentation. C, a still later stage of segmentation, /f,the smaller celU are growing over the larger cells but are involuted along acrescehtic line bl^ the dorsal lips of the blastopore. /, external view of theblastula, showing the circular blastopore. sion is described as meridional. Each upper pigmentedblastomere is divided into two longitudinally, an
An introduction to the study of the comparative anatomy of animals . , Forty-eight-cell stage, showing the smaller cells at the upper pole andlarger yoke-laden cells at the lower pole. £!, F, two sides of the same ovumin later stages of segmentation. C, a still later stage of segmentation, /f,the smaller celU are growing over the larger cells but are involuted along acrescehtic line bl^ the dorsal lips of the blastopore. /, external view of theblastula, showing the circular blastopore. sion is described as meridional. Each upper pigmentedblastomere is divided into two longitudinally, and the division Digitized by Microsoft® 124 COMPARATIVE ANATOMY lines are continued downwards into the yolk these, being so largely composed of inert deutoplasm,divide slowly, and their division is completed some timeafter it has been effected in the upper blastomeres. Thenext divisions may be described as equatorial, and affectall the sixteen blastomeres, dividing each of them into two,as is shown in fig. 26, C. The embryo now consists of thirty-. Fig. 27. A, vertical section through a segmenting ovum at about the stagerepresented in fig. 26 D. B, C, and jO, similar sections throughlater stages. Bit segmentation cavity or blastoccele, d^, blastopore.(After Morgan.) two cells, and the succeeding divisions become irregular, andare no longer synchronous. The upper pigmented cells dividemuch faster than the lower yolk-cells, and the final result ofthe first phase of development (usually called the segmentation-phase) is a vesicle containing a rather flattened and eccentriccavity, whose roof is composed of two layers of small pig- Digitized by Microsoft® DEVELOPMENT OF THE FROG 125 merited cells, and the floor of much larger irregularly-poly-gonal yolk-cells which occupy the whole of the lowerhemisphere of the embryo. The small cells pass, without anysharp line of demarcation, into the yolk-cells. Such a hollowvesicle is known as a blastula, and its cavity is know
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Keywords: ., bookcentury1900, bookdecade1900, bookpublisherlondo, bookyear1900
