. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES dysentery, passes from the alimentary canal of man safely protected by a thick, hyaline coat, until such time as ingestion by a suitable host brings about its dissolution in the host's stomach. The eggs of some of the tapeworms and roundworms are surrounded by dense impervious shells, rendering them viable, in the of Ascaris, for five or six years. Some of the parasitic roundworms are ovovivipa- rous, retaining the eggs within the body of the parent until thoy are nearly ready to hatch. A few fishes, like some of th
. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES dysentery, passes from the alimentary canal of man safely protected by a thick, hyaline coat, until such time as ingestion by a suitable host brings about its dissolution in the host's stomach. The eggs of some of the tapeworms and roundworms are surrounded by dense impervious shells, rendering them viable, in the of Ascaris, for five or six years. Some of the parasitic roundworms are ovovivipa- rous, retaining the eggs within the body of the parent until thoy are nearly ready to hatch. A few fishes, like some of the skates, produce an egg surrounded by a hard, leatherlike , which is drawn out into entangling tendrils that readily become enmeshed in seaweeds, thus affording protec- tion to the egg. Most of the fresh-water fishes and amphibians, how- ever, lay eggs which are protected by nothing more than a gelatinous mass which .swells after the eggs are laid in the water and are fertilized by the sperm. Among the reptiles and birds a hard shell is usually produced which gives protection to the enclosed ovum with its stored food. Only one small group of mammals, the monotremes, lay eggs, all others being viviparous. The Yolk Sac Among the fishes which lay telolecithal eggs containing enough yolk to render the cleavage pattern irregular, a mass of undivided yolk accumulates beneath the developing embryo. Soon, however, the blastoderm upon which the embryo lies grows down over the yolk, eventually enclosing it. This mass of tissue is composed of an inner layer of endoderm and an outer lining of mesoderm and is called H. w. H. — 28. Embryo and egg case of skate. Such cases afford protection against wave action. What other types of adaptations are there for the protection of eggs and embryos.^ (After Walker.). 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
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