The development of the human body; a manual of human embryology . Fig. 18 18.—Four Stages in the Segmentation of the Blastodermof the Chick.—(Coste.) nutritive material is so great that the segmentation isconfined to the small yolkless disk of protoplasm andaffects consequently only a portion of the entire distinguish this form of segmentation from that whichaffects the entire ovum it is termed meroblastic segmenta-tion, the other form being known as 58 THE DEVELOPMENT OF THE HUMAN BODY. In the ovum of a turtle or a bird the first plane of seg-mentation crosses the protop
The development of the human body; a manual of human embryology . Fig. 18 18.—Four Stages in the Segmentation of the Blastodermof the Chick.—(Coste.) nutritive material is so great that the segmentation isconfined to the small yolkless disk of protoplasm andaffects consequently only a portion of the entire distinguish this form of segmentation from that whichaffects the entire ovum it is termed meroblastic segmenta-tion, the other form being known as 58 THE DEVELOPMENT OF THE HUMAN BODY. In the ovum of a turtle or a bird the first plane of seg-mentation crosses the protoplasmic disk, dividing it intotwo practically equal halves, and the second plane formsat approximately right angles to the first one, dividingthe disk into four quadrants (Fig. 18, A). The thirddivision, like the two which precede it, is radial in position,while the fourth is circular and cuts off the inner ends ofthe six cells previously formed (Fig. 18, D). The disknow consists of six central smaller cells surrounded by six. Fig. 19.—Diagram Illustrating a Section of the Ovum of aReptile at a Stage Corresponding to the Blastula of anAmphibian. bl, Blastoderm; Y, yolk-mass. larger peripheral ones. Beyond this period no regularitycan be discerned in the appearance of the segmentationplanes; but radial and circular divisions continuing toform, the disk becomes divided into a large number ofcells, those at the center being much smaller than thoseat the periphery. In the mean time, however; thesmaller central cells have begun to divide in planesparallel to the surface of the disk, which, from being asimple plate of cells, thus becomes a discoidal cell-mass. SEGMENTATION OF THE OVUM. 59 During the segmentation of the disk it has increasedmaterially in size, extending further and further over thesurface of the yolk, into the substance of which some ofthe lower cells of the discoidal cell-mass have comparison of the diagram (Fig. 19) of the ovum ofa reptile at about
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
