Download e-book for iPad: A Synopsis of Biology by W. B. Crow (Auth.)

By W. B. Crow (Auth.)

ISBN-10: 1483200299

ISBN-13: 9781483200293

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OOGAMY IN ANIMALS The fusion of specialized gametes : s p e r m a t o z o a or sperm of the male (Fig. 16) and ova or eggs of the female in Parazoa and Metazoa. Fusion of the two is termed fertilization (also applied to plants). The spermatozoon differs from the ovum in (i) its smaller size, (2) its relatively small proportion of cytoplasm, (3) its flagellum, propelling it from behind (pulsellum), (4) its bilateral symmetry, and (5) in being produced in much larger numbers. The secretion of the glands connected with the male apparatus produces liquids in which the sperm may swim, chemical substances activating the sperm.

Experiments with chlamydomonads suggest a nutritive origin. These organisms in nutritive solutions reproduce asexually. Transferred to distilled water they produce gametes and fusion occurs. I t is considered t h a t fusions here take the place of the nutritive medium. The following sexual conditions are met with in organisms :— AGAMOGAMY The condition of being without sex. , derived by loss of the sexual condition. I t is also found in one of the generations of those organisms showing alternation of generations.

Polyzoa reproduce largely by e x t e r n a l b u d s , b u t in the order t h a t lives in fresh water (Phylactolaemata) there are i n t e r n a l b u d s or s t a t o b l a s t s (lens-shaped masses of cells, enclosed by chitinous shells) ; these alone survive the winter. Like the gemmules of sponges they are adaptations to freshwater life, their relatives, marine, in both cases not needing them. STABILIZATION External budding also occurs in ccelenterates and tunicates where it takes place usually by the separation of an individual formed as a l a t e r a l outgrowth.

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A Synopsis of Biology by W. B. Crow (Auth.)


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