. Outlines of zoology. Zoology. 262 CRUSTACEA. 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 are frequently found; these are the so-called yolk nuclei. Such a type of segmentation is called peripheral or cerilro- lecithal, and is very characteristic of Arthropod eggs. Over a particular r
. Outlines of zoology. Zoology. 262 CRUSTACEA. 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 are frequently found; these are the so-called yolk nuclei. Such a type of segmentation is called peripheral or cerilro- 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,. Fig. iit.—Longitudinal section of later embryo of Astacus.—After Reichenbach. Ec, Ectoderm; ;«., mesoderm cells; , cerebral ganglia; s/., stomodseuin; A., anus; telson; g., ventral ganglia; , Mernulsinus ; pd., proctodasum ; h., heart; mg., mid-gut; yolk pyramids dark. 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 into themselves in a way which early suggests their future digestive function. On the surface of the egg there have aheady appeared ectodermic thickenings,—the so-called eye-folds,—rudiments of the appendages, and of the thoracic and abdominal regions. In the later stages invaginations of the ectoderm form the fore- and hind-gut, which grow inward from opposite ends to meet the endoder-. 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. Edinburgh, Glasgow and London, H.
Size: 1589px × 1571px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
