. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. I] FORMATION OF THE SEX-CELLS 13 fereiitly from the rest, and are spoken of as the yolk-nuclei. They seem, in some way not fully understood, to be con- nected with the development of the yolk-granules.^ The yolk-granules, at first small and scattered, grow larger and become more numerous (Fig. 8). Before the egg leaves the ovary, the nucleus wanders toward the periphery and places itself under the black pole of the egg (Fig. 9). When the surface of the egg is examined, it shows a lighter area, owin
. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. I] FORMATION OF THE SEX-CELLS 13 fereiitly from the rest, and are spoken of as the yolk-nuclei. They seem, in some way not fully understood, to be con- nected with the development of the yolk-granules.^ The yolk-granules, at first small and scattered, grow larger and become more numerous (Fig. 8). Before the egg leaves the ovary, the nucleus wanders toward the periphery and places itself under the black pole of the egg (Fig. 9). When the surface of the egg is examined, it shows a lighter area, owing to the displacement of the pigment-granules in the region occu- pied by the nucleus. The nucleoli at this time migrate toward tlie centre of the nucleus,^ there disintegrate, and finally disappear. The chromatin-mate- rial draws together at this time into Fig. 9. — Section of ripe ova- , , , . , 1 1 ii rian egg. (After Hertwig.) threads which stain more deeply, the nuclear membrane disappears, an achromatic spindle develops, and the egg is ready to extrude the first polar body. Comparison of Spermatogenesis with Oogenesis The method of extrusion of the polar bodies is described in the next chapter, but we may anticipate this account in order to consider here a remarkable parallel that has been discovered between the formation of the polar bodies and the formation of the spermatozoa. In the latter, as we have seen, two successive divisions follow each other during the maturation-period tvith- out an mtervening re sting-stage. The tetrad-groups are present at the beginning of the process. After the two maturation- divisions the number of chromosomes is reduced to half the number characteristic for the species.^ The same phenomena appear when the polar bodies are extruded from the egg. After the extrusion of the first polar body, the spindle for the second 1 Will ('84) describes the yolk-nuclei as arising from constrictions of the nucleus set free with their nucleoli into the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectembryology, bookyear1
