Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 rERTILIZATION. 109 the protoplasm of the ovum, is thrown out of the egg as the so-called directive bodies or 2}olar cells (fig. 101). The part of it, however, which remains in the ovum retains its significance as a nucleus, and is known as the female pronucleus. This fu-ies with the single spermatozoon (male pronucleus) which has forced its way into the ovum (fig. 102); and the compound structure so formed constitutes the nucleus of the
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 rERTILIZATION. 109 the protoplasm of the ovum, is thrown out of the egg as the so-called directive bodies or 2}olar cells (fig. 101). The part of it, however, which remains in the ovum retains its significance as a nucleus, and is known as the female pronucleus. This fu-ies with the single spermatozoon (male pronucleus) which has forced its way into the ovum (fig. 102); and the compound structure so formed constitutes the nucleus of the fertilized ovum, or as it is generally called, the first segmentation nucleus. Fig. 103.—Development of a Star-fish, Asteracanthion berylinus (after Alex. Agassiz). 1, Commencing segmentation of the flattened egg—at one pole are seen the polar bodies ; 2, stage with two segments ; 3, with four ; 4, with eight; 5, with thirty-two segments ; 6, later stage; 7, blastosphere with commencing invagination; 8 and 9, more advanced stages of invagination. The opening of the gastrula cavity becomes the aims. This new nucleus, which di\'ides to give rise to the nuclei of the first segmentation spheres, would appear therefore to be the product of the fusion or conjugation of the part of the germinal vesicle, which remains behind in the ovum, with the male pronucleus, which is a derivative of the spermatozoon which has entered the ovum. Fertilization would appear, therefore, to depend upon the addition
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