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Figure 4–3. Fresh-water hydra (Hydra vulgaris): a — general appearance; b — budding ; c — sexually mature hydra with eggs (From Life of Animals, vol. 1, 1968).
However, from genetical point of view one cannot pass over a very
important question related to the development of this process: to what
extent is the autonomy of the cells, building multicellular organisms,
preserved and is there a clearly expressed dividing line between the
constituent cells and the organisms as a whole. It may well be that many
processes, phenomena, troubles originate from the condition and activity of
a cell or group of cells. The elucidation of this question is forthcoming.
In the initial stages of evolution the formed multicellular organisms
reproduce asexually by forming new coenobia and colonies, or by budding.
Such capabilities can be traced in volvox algae, sea fungi, hydra, etc. In the
course of complicating their organization, along with the asexual
reproduction, the sexual process arises. In higher organisms, including
man, evolution differentiates the two types of cells — vegetative (somatic)
and sexual (reproductive).
As it was already mentioned, in multicellular organisms the vegetative or
somatic cells normally posses a diploid set of chromosomes (2n), and the sexual
ones — haploid (1n). To develop a new organism, it is necessary two sexual
cells (male and female) to fuse and form a zygote in which the genomes of the
initial parent organisms are combined. This is the biological sense of the sexual
reproduction irrespectively of its variety regarding ways and forms of realization.