. An elementary course of practical zoology. Zoology. VI Development 369' themselves : they are feilihzed immediately after being laid, the male depositing spermatophores on the ventral surface of the female's body just before oviposition. The process of segmentation of the oosperm presents certain striking peculiarities. The nucleus divides rejjeatedly (l*'ig. 91 A, IIli), but no corresponding division of the pro- toplasm takes place, with the result that the polyplast-stage (p. 200), instead of being a heap of cells, is simply a multi- nucleate body (compare p. 269). Soon the nuclei thus for
. An elementary course of practical zoology. Zoology. VI Development 369' themselves : they are feilihzed immediately after being laid, the male depositing spermatophores on the ventral surface of the female's body just before oviposition. The process of segmentation of the oosperm presents certain striking peculiarities. The nucleus divides rejjeatedly (l*'ig. 91 A, IIli), but no corresponding division of the pro- toplasm takes place, with the result that the polyplast-stage (p. 200), instead of being a heap of cells, is simply a multi- nucleate body (compare p. 269). Soon the nuclei thus formed retreat from the centre of the embryo, and arrange themselves in a single layer close to the surface (d) : around. 7W Vu,. gi.—Two stages in the early development of tlie Crayfish. Ill pL the products of division of the nucleus {iitt) are seen in the centre of the yolk ; in ]J the nuclei have become arranged in a' peripheral layer, each sur. rounded by protoplasm, so as to form the blastoderm. (From Parker and Haswell's Zoology^ after Worin.) each of these protoplasm accumulates, the central part of the embryo consisting entirely of yolk-material. '\\^e thus get a. superficial segineittatioii, 'Aitd by a central mass of yolk and a superficial layer of cells collectively known as the blastoderm. From this the ectoderm and endoderm are derived, the latter enclosing a relatively small enteron com- municating with the exterior through a blastopore (p. 201). Very soon the embryo becomes triploblastic, or three- layered, by the budding off of cells from the endoderm in the neighbourhood of the blastopore : these accumulate between the ectoderm and endoderm, and constitute the mesoderm. • Pkact. Zool. B B. 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 Parker, T. Jeffery (Thomas Jeffery), 1850-189
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1900
