. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . g unitedin the one cell whichforms the wholebody of the or-ganism. In the Met-azoa, however, thisdifferentiation hastaken place; the germinal cells are set apart for the preservation of therace; the somatic cells carry on their various functions for a time,grow old, die, and disappear, certain of the germ cells alone survivingin the production of new individuals. On the borderland betweenthe unicellular and the multicellular organisms, however, stand cer-
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . g unitedin the one cell whichforms the wholebody of the or-ganism. In the Met-azoa, however, thisdifferentiation hastaken place; the germinal cells are set apart for the preservation of therace; the somatic cells carry on their various functions for a time,grow old, die, and disappear, certain of the germ cells alone survivingin the production of new individuals. On the borderland betweenthe unicellular and the multicellular organisms, however, stand cer-tain colonial forms, which show an exquisitely graded series of steps,from the conditions of unicellular multiplication to those of the multi-cellular forms. (McFarland.) In the many-celled animals the egg is a single cell ladenwith a large amount of food yolk, and made up of nucleus andcytoplasm as the living elements. For the normal developmentof this egg, conjugation with another germ cell, derived from adifferent individual, is usually necessary. This germ cell is thespermatozooid, a minute cell consisting of nucleus and centro-18. FIG. 147.—Gonium perforate, a simple colonial Protozoan,composed of sixteen cells holding together in a singlelayer or plate: A, The whole colony; B, a single cell; e,eye spot; c/, chloroplast; n, nucleus; v, vacuole. (AfterCampbell.) 262 INVOLUTION AND ANIMAL LIFE some with a small amount of cytoplasm modified primarily intoan organ of locomotion, the tail. A physiological division oflabor is here met with which admirably meets two diametricallyopposed requirements. The one of these demands that theconjugating cells be highly motile, and consequently small,in order that they may be able to come together in the waterin which they are usually set free. The second requires thatthere be furnished a sufficient amount of nutritive materialfor the nourishment of the embryo until it arrives at a stage ofgrowth in which it can shift for itself. These t
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