(f) But whether there is insemination or not, there is in the great majority of animals the essential process of fertilization, the intimate and orderly union of the sex-cells. (For exceptions, see PARTHENOGENESIS.) As the nuclei of the ripe gametes have under gone meiotic division, the process of fertilization restores the number of chromosomes to the normal. It also implies the union of the paternal and maternal hereditary factors, a stimulus to the egg-cell to divide, and a blocking of the egg-cell against other sperms. (See FERTILIZATION.) In flowering plants the process of pollination would roughly correspond to insemination in ani mals, while the union of the microscopic male nucleus from the pollen-tube with the microscopic female nucleus in the egg-cell within the ovule's embryo-sac, is the act of fertilization. In flowerless plants and in the primitive flowering plants known as Cycads and Gingkos, the male cell is a locomotor sperm (anther ozoid) as in most animals.
(g) Development (see EMBRYOLOGY)—the process by which the fertilized ovum builds up an embryo—is a study by itself yet it cannot be rigidly separated off from reproduction, for it is through development that the organization of the parents is re produced. Moreover, there is the peculiar occurrence of poly embryony in some armadillos, e.g., Dasypus novemcinctus, and in some parasitic Hymenoptera, e.g., Encyrtus, where the developing egg normally produces several embryos, which is obviously a pro cess of multiplication. It would also be pedantic to try to ex clude from the rubric of reproduction the various ways in which the maternal parent contributes to the development of the off spring while it remains within her body. In the gestation of ordi nary mammals the placenta establishes what may be called a symbiotic relation between the mother and the offspring; and of this, as Aristotle knew, there are anticipations even at the level of dogfishes. Hints of ante-natal linkage and nutrition are also seen in a few reptiles and even in the primitive Onychophora (q.v.). (h) The study of reproduction must also join hands with the study of heredity,—the relation of organic continuity be tween successive generations, which tends to secure the begetting of like by like ; and this is particularly the case when there is an intimate linkage between mother and offspring (or between the plant and its seed!) supplying the early "nurture" required if the hereditary "nature" is to express itself aright. (i) Nor can we exclude a consideration of the diverse ways in which the new generation is separated from the old, whether by buds and frag ments, or by ova, or by ova that have developed before libera tion into larvae or into miniature organisms. And this may be complicated by the occurrence of alternation of generations. (j) Finally, biology cannot be content with a study of reproduc tion as a problem in the physiology of the individual, or even of the pair. There is a higher physiology or ecology of reproduction,
which concerns itself with such questions as monogamy, polygyny and polyandry (as in the cuckoo) and with the different forms of animal family (see SOCIOLOGY, ANIMAL).
Modes of Reproduction in Unicellular Organisms.— Among one-celled organisms, whether protists, Protophyta, or Protozoa, the unit divides into two or more parts (by equal fis sion, or by giving off small buds, or by spore-formation) ; and each of these parts grows into the likeness of the whole. In the great majority of cases, the division of the unicellular organism is preceded by the division of the nucleus, and in some types this division takes the form of intricate mitosis (q.v.). Yet there are some cases, such as the production of multiple buds around the margin of Arcella, or the very rough-and-ready fragmentation de scribed in Schizogenes, where reproduction does not seem to be far removed from rupture. In all cases of division there is prob ably some plasmic instability within the cell, which leads to cy tolysis; and it was perhaps one of the early tasks of organic evo lution, so to speak, to regularize this disintegration, so that it led not to death, but to more life. The plasmic instability, which modern biochemistry and biophysics are seeking to define, may be the natural outcome of growth. For it was pointed out by Herb ert Spencer and Rudolf Leuckart that a growth-increase in the volume of living matter is not accompanied in regular shapes by a pro tanto increase in the surface by which the processes of keeping alive are effected. Thus in spheres, the volume increases as the cube of the radius, while the surface increases only as the square. The consequences of this may be partly evaded, as in Rhizopoda which acquire a large surface by flowing out in numer ous pseudopodia ; but the general idea is sound. And in addition to the volume-surface ratio, there is the relation (emphasized by R. Hertwig) between nucleoplasm and cytoplasm, which seems to have its optimum and its limits. When the cell divides, there is a reduction of volume, but a relative increase of the surface. Or there may be an adjustment of the ratio between nucleoplasm and cytoplasm ; and in this connection the not infrequent occur rence of multiple nuclei should be considered. Of interest are the experiments of Gruber who showed that excised non-nucleated fragments of some Protozoa, which may live for a time and even show growth and repair, eventually fail to survive, the nucleo plasm, in some form or other, being apparently essential. It is suggested, then, that the beginnings of multiplication are to be looked for in cell-rupture following instability.