The development of the human body; a manual of human embryology . -tained a considerable amount of yolk, many of the pecu-liarities resulting from its presence being still clearlyindicated in the early development of the almost yolklessmammalian ovum. To give some idea of the peculiaritieswhich result from the presence of considerable amounts ofyolk it will be well to compare the processes of segmenta-tion and differentiation seen in ova with different amountsof yolk. A little below the scale of the vertebrates proper is aform, Amphioxus, which possesses an almost yolkless 54 SEGMENTATION OF T
The development of the human body; a manual of human embryology . -tained a considerable amount of yolk, many of the pecu-liarities resulting from its presence being still clearlyindicated in the early development of the almost yolklessmammalian ovum. To give some idea of the peculiaritieswhich result from the presence of considerable amounts ofyolk it will be well to compare the processes of segmenta-tion and differentiation seen in ova with different amountsof yolk. A little below the scale of the vertebrates proper is aform, Amphioxus, which possesses an almost yolkless 54 SEGMENTATION OF THE OVUM. 55 ovum presenting a simple process of development. Thefertilized ovum of Amphioxus in its first division separatesinto two similar and equal cells, and these are made four(Fig. 16, A) by a second plane of division which cuts theprevious one at right angles. A third plane at right anglesto both the preceding ones brings about an eight-celledstage (Fig. 16, B), and further divisions result in the for-mation of a large number of cells which arrange them-. Fig. 16.—Stages in the Segmentation of Amphioxus. A, Four-celled stage; B, eight-celled stage; C, sixteen-celled stage; D, early blastula; E, blastula; F, section of blastula.—(Hatscheh.) selves in the form of a hollow sphere which is known as ablastula (Fig. 16, E). The minute amount of yolk which is present in theovum of Amphioxus collects at an early stage of the seg-mentation at one pole of the ovum, the cells containing itbeing somewhat larger than those of the other pole (, B), and in the blastula the cells of one pole are largerand more richly laden with yolk than those of the otherpole (Fig. 16, F). If, now, the segmenting ovum of anAmphibian be examined, it will be found that a very 56 THE DEVELOPMENT OF THE HUMAN BODY. much greater amount of yolk is present and, as in Am-phioxus, it is located especially at one pole of the first three planes of segmentation have the samerelative positions as
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
