. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. REPRODUCTION. 191 atricula, yolk, and vitelline membrane. The shell and albumen are often wanting. When the albumen is pres- ent, it is commonly covered by a membrane only, as in \jjl£i c' ^ z)--'- ^. Frogs; in Sharks, the envelope is horny; and in Croco- diles it is calcareous, as in Birds. The eggs of all animals, in their essentials, are alike in kind, but not in degree. The ^gg of the Amoeba is a mere cell, with a light S2')0t in one part of the contents. The Qgg of the Hydra differs in having this
. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. REPRODUCTION. 191 atricula, yolk, and vitelline membrane. The shell and albumen are often wanting. When the albumen is pres- ent, it is commonly covered by a membrane only, as in \jjl£i c' ^ z)--'- ^. Frogs; in Sharks, the envelope is horny; and in Croco- diles it is calcareous, as in Birds. The eggs of all animals, in their essentials, are alike in kind, but not in degree. The ^gg of the Amoeba is a mere cell, with a light S2')0t in one part of the contents. The Qgg of the Hydra differs in having this light spot sharply defined, and inclosed in a sac, or gerininal vesi- cle. In the Rabbit's ^%^^ thei-e is another sac, called ger- minal dot, inside the germinal vesicle; while the eggs of most of the higher animals show a nucleolus within the germinal dot. An ^gg, therefore, closely resembles a cell, consisting of an outer and an inner sac and a dot, or a series of hollow concentric spheres. The size of an e,^^^^ depends mainly upon the quantity of 3'olk it contains; and this is proportioned to the grade of development which the embryo attains when it leaves the ;* In the eggs of the Star-fishes, "Worms, Insects, Mollusks (except the Cuttle-fishes), and Mammals, the yolk is very minute and formative, , it is converted into the parts of the future embryo. In the eggs of Lobsters, Crabs, Spiders, Cephalopods, Fishes, Eeptiles, and Birds,. 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 Orton, James, 1830-1877. New York, Harper and brothers
Size: 2127px × 1174px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, bookpublishe, booksubjectzoology
