Quain's elements of anatomy . given at p. G98 of this volume. Introdtiction of the Spermatozoa into the Ovum.—The factof the actual entrance of spermatozoa within the zona or covering ofthe mammiferous ovum was first observed by Martin Barry in 1843,and although his statement was received with considerable hesitationby his contemporaries, it has since been repeatedly confirmed by theminute and careful investigation of many observers. In certain animals the spermatozoa have been seen to enter the cavityof the oxmn. by an obvious micropyle aperture, as first observed byRansom in fishes (No. 72),
Quain's elements of anatomy . given at p. G98 of this volume. Introdtiction of the Spermatozoa into the Ovum.—The factof the actual entrance of spermatozoa within the zona or covering ofthe mammiferous ovum was first observed by Martin Barry in 1843,and although his statement was received with considerable hesitationby his contemporaries, it has since been repeatedly confirmed by theminute and careful investigation of many observers. In certain animals the spermatozoa have been seen to enter the cavityof the oxmn. by an obvious micropyle aperture, as first observed byRansom in fishes (No. 72), and by jMeissner and Leuckart in insects ; butin manmials and other animals in which no such aperture exists, it isnot yet clearly understood in what manner the spermatozoa make then-v\ay through the consistent membrane of the ovum. Changes in the Ovum and Spermatozoa giving nse to -a,Male Pronucleus.— In mammalia, although the spermatozoa areascertained to pass in numbers through the zona, they have not been. Fig. ^. h. g^p: Fig. 628.—Fertilization of the ovum of an ECHiNODEEM(Selenka.) s, spermatozoon ; tn.^rr, male pronucleus ; f-l^r, female pronucleus. 1. Accession of a spermatozoon to the periphery of the vitellus ; 2. Its penetiation, andthe radial disposition of the vitelline granules ; 3. Transformation of the head of the sper-matozoon into the male pronucleus ; 4, 5. Blending of the male and feiaale xu-onuclei observed in any case to penetrate or combine with the substance of theyolk. It is extremely probable that such combination does take place,but we only know of this occurrence by observations made on the loweranimals, which, however, throve so much light on the whole process offecundation that it is proper to give an account of them in this place. 3 B 2 740 THE OVUM BEFOEE DEVELOPMENT. In this account we shall follow mainly as before the statements of Foland Hertwig, together with some others as described in Balfours veryclear account of this subject.
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