. Outlines of zoology. Zoology. DEVELOPMENT. 267 Arthropoda. The spherical ovum is surrounded by a cuticular vitelline membrane, and contains a considerable quantity of yolk. After ferti- lisation the segmentation nucleus divides in the usual way into 2, 4, 8, and so on, but this nuclear division is not followed by division of the plasma. Eventually the nuclei, each surrounded by a small amount of protoplasm, approach the surface of the egg and arrange themselves regularly round it. The peripheral protoplasm then segments round these nuclei, and thus we have a central core of unsegmented yolk
. Outlines of zoology. Zoology. DEVELOPMENT. 267 Arthropoda. The spherical ovum is surrounded by a cuticular vitelline membrane, and contains a considerable quantity of yolk. After ferti- lisation the segmentation nucleus divides in the usual way into 2, 4, 8, and so on, but this nuclear division is not followed by division of the plasma. Eventually the nuclei, each surrounded by a small amount of protoplasm, approach the surface of the egg and arrange themselves regularly round it. The peripheral protoplasm then segments round these nuclei, and thus we have a central core of unsegmented yolk enveloped by a peripheral ring of rapidly dividing cells. In the central yolk free nuclei may be frequently found, these are the so-called yolk. Fig. 87.—Longitudinal section of later embryo of Astacus. (After Reichenbach.) ec, Ectoderm ; in, mesodernt cells ; eg, cerebral ganglia ; st, stoma- todaeum ; A, anus : T, Lelson; ff, ventral ganglia ; ss, sternal sinus ; id, proctodseum ; h, heart; 7ug, mid gut; yolk pyramids (dark). nuclei. Such a type of segmentation is called peripheral or centro- lecithal, and is very characteristic of Arthropod eggs. Over a particular region of the segmented egg, known as the " ventral plate," the cells begin to thicken ; at this region an invagination occurs, which represents the gastrula. At the anterior lip of the blastopore the mesoderm appears, being many celled from the first. Soon the blasto- pore closes; the cavity of the gastrula thus becomes a closed sac—the future mid gut. The cells of this archenteron take up the core of yolk. 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 Thomson, J. Arthur (John Arthur), 1861-1933; Newbigin, Marion I. (Marion Isabel), 1869-1934; Smith, Preserved, 1880-1941. fmo. New York, D. Appleton & Co.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
