. The cell in development and inheritance. Cells. Fig. 72. — Origin of the germ-cells in a hydro-medusa, Cladonema. [] A. Young stage; section through wall of manubrium of the medusa; ova developing in the ectoderm (ec). B. Later stage, showing older ova (o) and "nutritive cells" («). The ova contain small nuclei probably derived from engulfed nutritive cells. two primordial germ-cells lying at the apex of the archenteron. In some of the insects they appear still earlier as the products of a large " pole-cell " lying at one end of the segmenting ovum, which divide
. The cell in development and inheritance. Cells. Fig. 72. — Origin of the germ-cells in a hydro-medusa, Cladonema. [] A. Young stage; section through wall of manubrium of the medusa; ova developing in the ectoderm (ec). B. Later stage, showing older ova (o) and "nutritive cells" («). The ova contain small nuclei probably derived from engulfed nutritive cells. two primordial germ-cells lying at the apex of the archenteron. In some of the insects they appear still earlier as the products of a large " pole-cell " lying at one end of the segmenting ovum, which divides into two and finally gives rise to two symmetrical groups of germ- cells. Hacker has recently traced very carefully the origin of the primordial germ-cells in Cyclops from a "stem-cell" (Fig. 74) clearly distinguishable from surrounding cells in the early blastula stage, not only by its size, but also by its large nuclei rich in chromatin, and by its peculiar mode of mitosis, as described beyond. The most beautiful and remarkable known case of early differenti- ation of the germ-cells is that of Ascaris, where Boveri was able to trace them back continuously thiough all the cleavage-stages to the. 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 Wilson, Edmund B. (Edmund Beecher), 1856-1939. New York Macmillan
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1906
