. The biology of the amphibia. Amphibians. DEVELOPMENT AND HEREDITY 17 capsules overlying a depression in the surface of the egg by their digestive powers. Why the spermatozoa accumulate only in the region of this depression has not been determined. Miss Hibbard suggests that the nuclear fluids which collect at the bottom of the depression may exert a chemotactic effect on the sperm. In the common frog, Rana, although a much higher type than Discoglossus, there is less localization of the area of penetration. The first spermatozoon to reach the darker hemisphere of the egg sets up a fertilizat
. The biology of the amphibia. Amphibians. DEVELOPMENT AND HEREDITY 17 capsules overlying a depression in the surface of the egg by their digestive powers. Why the spermatozoa accumulate only in the region of this depression has not been determined. Miss Hibbard suggests that the nuclear fluids which collect at the bottom of the depression may exert a chemotactic effect on the sperm. In the common frog, Rana, although a much higher type than Discoglossus, there is less localization of the area of penetration. The first spermatozoon to reach the darker hemisphere of the egg sets up a fertilization reaction. After it. Fig. 7.—Spermatophores of common salamanders. A. Triturus viridescens (after Smith). B. Desmognathus fuscus (after Noble and Weber). C. Eurycea bislineata. has entered the egg, the latter forms a fertilization membrane which prevents the entrance of other spermatozoa. In Caudata several spermatozoa normally enter the egg but only one sperm nucleus combines with the egg nucleus, the others degenerating before segmentation is far advanced. The number of sper- matozoa which may safely enter the urodele egg, without causing irregularities of development leading to death, stands roughly in proportion to the size of the egg. Polyspermy obtains among eggs, such as those of Cryptobranchus, in which the mass of yolk is considerable. It seems to be a device for large eggs, insuring that one sperm at least shall enter at a point near the egg nucleus. Fertilization includes two processes: activation or the removal of the block to development, and syngamy or the union of the nucleus of the egg with that of the spermatozoon. The first process may be induced artificially in frogs by pricking the egg. 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 Noble, Gladwyn Kingsley, 1894-1940. New York : McG
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
