Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/704

686 isms ; so that, physiologically, as well as morphologically, development is a progress from the general to the special. Thus far, the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the influence of the &quot;conditions of existence&quot; thus deter mined constitutes a most important part of Physiology.

The sustentative functions, for example, can only be exerted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive &quot;struggle for existence.&quot; The exercise of the correlative functions is influenced by similar condi tions, and by the direct conflict with other organisms, which constitutes the active struggle for existence. And, finally, the generative functions are subject to extensive modifi cations, dependent partly upon what are commonly called external conditions, and partly upon wholly unknown agencies.

In the lowest forms of life, the only mode of generation at present known is the division of the body into two or and more parts, each, of which then grows to the size and assumes the form of its parent, and repeats the process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are severally competent to give rise to individual organisms of the same nature as that from which they arose.

In many of the lowest organisms the process is modified so far that, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as a bud which develops into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation are not confined t&amp;lt;- the simplest forms of life, however. On the contrary, both modes of multipli cation are common not only among plants, but among animals of considerable complexity. The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polypes by budding, are well-known examples of these modes of reproduction. In all these cases, the bud or the segment consists of a multi tude of more or less metamorphosed cells. But, in other instances, a single cell detached from a mass of such un- differentiated cells contained in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment whether the process is more akin to fission or to gemmation. In all these cases the development of the new being from the detached germ takes place without the influence of other living matter. Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the way indicated above, ceases. At most, we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part ; and, in the very highest animals, even this power vanishes in the adult ; and, in most parts of the body, though the undifferentiated cells are capable of multi plication, their progeny grow, not into whole organisms like that of which they form a part, but into elements of the tissues.

Throughout almost the whole series of living beings, however, we find concurrently with the process of agamogenesis, or asexual generation, another method of genera tion, in which the develoDment of the germ into an 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 many exceptions in detail, it may be said that there is an inverse relation between agamogenetic and garnogenetic reproduction. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogencsis is absent. In many of the lower forms of life agamogenesis is the common and predominant modo 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 &quot; conjugation&quot; sexual generation consists in the coalescence of two similar masses of protoplasmic matter, derived from different parts of the same organism, 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 cell, to which a greater or less amount of nutritive material, constituting the food-yolk, 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 Algce 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 usu ally are in active vibratile movement, and sometimes are pro pelled 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 the substance of the embryo cell, have been discovered ; and there is the same difficulty with respect to the conjugative processes of. some of the Gryptogamia. 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 pro cess 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.

The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (blastomeres), which may remain united as a single cell-aggregate, or some or all of which may become separate organisms. A larger or shorter period of rest, in many cases, intervenes between the act of impregnation and the commencement of the process of division. As a general rule, the female cell which directly receives the influence of the male is that which undergoes division and eventual development into independent germs ; but there are some plants, such as the Florideoe, in which this is not the case. In these the protoplasmic body of the trichogyne, which unites with the molecular spermatozoids, does not undergo division itself, but transmits some influ ence to adjacent cells, in virtue of which they become subdivided into independent germs or spores. There is still much obscurity respecting the reproductive processes of the Infusoria ; but, in the Vorticellidce, it would appear that conjugation merely determines a condition of 