Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 \ /JS^^I\ 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 bec
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 \ /JS^^I\ 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 anus. This new nucleus, which divides 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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