Sexual Reproduction (Gamogenesis)
Throughout almost the whole series of living beings, however, we find concurrently with the process of agamogenesis, or asexual generation, another method of generation, in which the development of the germ into a organism resembling the parent depends on an influence exerted by living matter different from the germ. This is gamogenesis, or sexual generation. Looking at the facts broadly, and without reference to may exceptions in detail, it may be said that there is an inverse relation between agamogenetic and gamogenetic reproduction. In the lowest organisms gamogenesis is absent. In many of the lower forms of life agamogenesis is the common and predominant mode of reproduction, while gamogenesis is exceptional; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis takes place exceptionally. In its simplest condition, which is termed "conjugation," sexual generation consists in the coalescence of two similar masses of protoplasmic matter, derived from different parts of the same organisms, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism.
In the majority of cases, however, there is a marked morphological difference between the two factors in the process, and then one is called the male, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals it is a nucleated cells, to which a greater or less amount of nutritive material, consisting the food-yelk, may be added.
The male element, on the other hand, is relatively small. It may be conveyed to the female element by an out-growth of the wall of its cell, which is short in many Algae and Fungi, but becomes an immensely elongated tubular filament, in the case of the pollen cell of flowering plants. But, more commonly, the protoplasm of the male cells becomes converted into rods or filaments, which usually are in active vibratile movement, and sometimes are propelled by numerous cilia. Occasionally, however, as in many Nematoidea and Arthropoda, they are devoid of mobility.
The manner in which the contents of the pollen tube affect the embryo cell in flowering plants is unknown, as no perforations through which the contents of the pollen tube may pass, so as actually to mix with substance of the embryo cell, have been discovered; and there is the same difficulty with respect to the conjugative processes of some of the Cryptogamia. But in the great majority of plants, and in all animals, there can be no doubt that the substance of the male element actually mixes with that of the female, so that in all these cases the sexual process remains one of conjugation; and impregnation is the physical admixture of protoplasmic matter derived from two sources, which may be either different parts of the same organism, or different organisms.
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