. The physiology of reproduction. Reproduction. 190 THE PHYSIOLOGY OF REPRODUCTION normally after the addition of spermatozoa, while Driesch has shown that if such ova are deprived of their envelopes by shaking, and are then divided into fragments some of which contain no nuclei, the latter are capable of being ferti- Ksed a second time. It is clear, therefore, that in such cases the union of nuclei is not essential for the development of the In those ova which are surrounded by a membrane it is probable that the fertihsing spermatozoon bores its way through at any point (Mammals and Am
. The physiology of reproduction. Reproduction. 190 THE PHYSIOLOGY OF REPRODUCTION normally after the addition of spermatozoa, while Driesch has shown that if such ova are deprived of their envelopes by shaking, and are then divided into fragments some of which contain no nuclei, the latter are capable of being ferti- Ksed a second time. It is clear, therefore, that in such cases the union of nuclei is not essential for the development of the In those ova which are surrounded by a membrane it is probable that the fertihsing spermatozoon bores its way through at any point (Mammals and Am- phibians). In other cases there is a small aperture in the wall of the ovum; this is called the micropyle ' (some Pisces and Insecta). Some eggs, however, are naked, so that the sperms may effect an entrance anywhere on the surface (some Echinoderms and Cceleriterates), or there may be funnel-shaped depres- sions on the egg's periphery (certain hydromedusee). ^ Fig. 52.—Fertilisation process In the majority of animals only in bat's ovum. (After van one spermatozoon normally enters "° '' the ovum, but in some (certain p. 6., polar bodies ;<,.«., nucleus insects, elasmobranch fishes, reptiles, of ovum: s. n., nucleus of ,, , i .i n o » spermatozoon. earthworm, lamprey, axolotl,^ &c.), several may efiect an entrance. The latter condition is called Polyspermy. Only one sperm- nucleus conjugates with the ovum-nucleus; the others as a general rule undergo degeneration, but in a few cases (elasmobranchs and reptiles) they are said to divide, forming accessory nuclei whose ultimate fate is unknown. In those animals in which only one sperm normally enters the egg, pathological polyspermy may occasionally occur. In such cases each sperm centrosome may give rise to a sperm-aster. The • For references to the original papers, which are somewhat numerous, see Przibram, Embi-yogeny, English Translation, Cambridge, 1908. 2 Wilson, The Cell, &c., 2nd Edition, New York,
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