Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 112 ORGANIZATION AND DEVELOPMENT OF ANI3IAL3 IX GENERAL. of the original germinal cell, while the yolk is only secondarily developed with the gradual growth of the first; and not unfrequently it is derived from the secretion of special glands (yolk glands, Trema- todes); it may even be added in the form of cells. In the Ctenophora and other Coelenterata we see already in the first-formed segments the separation of the formative matter or peripheral ecto
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 112 ORGANIZATION AND DEVELOPMENT OF ANI3IAL3 IX GENERAL. of the original germinal cell, while the yolk is only secondarily developed with the gradual growth of the first; and not unfrequently it is derived from the secretion of special glands (yolk glands, Trema- todes); it may even be added in the form of cells. In the Ctenophora and other Coelenterata we see already in the first-formed segments the separation of the formative matter or peripheral ectoplasm from the nutritive matter or central endoplasm. In eggs undergoing a partial segmentation the formative matter usually lies on one side of the large unsegmenting food yolk. In accordance with this, the segments of such eggs, known as telolecitfttil, arrange themselves in the form of a flat disc (germinal disc); hence this kind of segmentation has been called discoidal (eggs of Aves, Reptilia, Pisces) (fig. 105). The food yolk may, however, have a central position. In such centrolecithal eggs the segmentation is FIG. 106.—Unequal segmentation of the centrolecithal egLT of Gammarus locusta (in part after Ed. van Beneden). The central yolk mass does not appear till a late stage and undergoes later an ' after-segmentation.' confined to the periphery, and is sometimes equal (Palsemon) and sometimes unequal (fig. 106). The central yolk mass may at first remain unsegmented, but later it may undergo a kind of after- segmentation and break up into a number of cells (fig. 106). Again, in other cases the food yolk, at the commencement of segmentation, has a peripheral position, so that the cleavage process is at first confined to the inner parts of the egg, and only in later stages, when the food yolk has gradually shifted into the centre of the egg, appears as a peripheral layer on the surface. This is found especially in the eggs of Spiders (fig. 107). The first processes of
Size: 2353px × 850px
Photo credit: © Bookend / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: archive, book, drawing, historical, history, illustration, image, page, picture, print, reference, vintage
