A manual of elementary zoology / by L.A. Borrodaile.

  • Lancelot Alexander Borradaile
Date:
1945
Catalogue details

Licence: In copyright

Credit: A manual of elementary zoology / by L.A. Borrodaile. Source: Wellcome Collection.

    35/840 (page 15)
    are known as gametes, and that which results from their fusion as a zygote. In some of the smallest living beings (Fig. 4) syngamy is the union ot fully-grown adults and has no connection with reproduction. In other such creatures (Fig. 10), however, and in all large and complex animals and plants, syngamy takes place only between the reproductive bodies, which are generally unable to develop without it, so that it becomes a necessary part of the reproductive process. In these cases the reproductive bodies are of a kind known as germs, distinguished from other reproductive bodies (free buds, etc.) by their small size and the simplicity of their structure. The germs of such creatures are usually •of two sizes which unite larger with smaller (Fig. 10 C). In all large and complex animals (and in some •of the smallest) the gametes differ in form and behaviour as well as in size (Figs. 2 and 5). One is larger and passive, and is called the female gamete, or, in large animals, the egg or ovum. The other is smaller and active, and known as the male gamete or spermatozoon ; it has usually a tail (flagellum) with which it swims in the fluid in which it is borne, and thus it moves to the •egg and enters the latter (Fig. 6). This process is known as the fer- tilisation of the ovum. After it the fertilised ovum or oosperm proceeds to develop. Ova and spermatozoa are usually formed by different adults, known respectively as female and male., but in some cases both kinds are formed by one individual, which is then known as a her- maphrodite. In some aquatic animals the gametes are set free, and syngamy takes place outside the body of the parent. In many cases, however, the ova are kept within Fig. 4. — Copromonas, a minute inhabitant of dung.—After Dobell. a, Adult individual ; b, the same in fission ; c, two adult in- dividuals in syngamy; d, the zygote, enclosed in a cyst. f.v., Food vacuole; fl., flagellum; g., gullet; nu., nucleus; res., reservoir of contractile vacuole (see p. 140).