Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/705

Rh the whole organism, which gives rise to the division of the endoplast or so-called nucleus, by which germs are thrown off; and if this be the case, the process would have some analogy to what takes place in the Floridece. On the other hand, the process of conjugation by which two distinct Diporpce combine into that extraordinary double organism, the Diplozoon paradoxum, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individuals ; and the same process takes place in a large number of the Infusoria, if what are supposed to be male sexual elements in them are really such. The process of impregnation in the Floridece is remark ably interesting, from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism other than the ovum, even in the highest animals and plants. The nature of the influence exerted by the male element upon the female is wholly unknown. N&quot;o morphological distinction can be drawn between those cells which are capable of reproducing the whole organism without im pregnation, and those which need it, as is obvious from what happens in insects, where eggs which ordinarily re quire impregnation, exceptionally, as in many moths, or regularly, as in the case of the drones among bees, develop without impregnation. Even in the higher animals, such as the fowl, the earlier stages of division of the germ may take place without impregnation. In fact, generation may be regarded as a particular case of cell multiplication, and impregnation simply as one of the many conditions which may determine or affect that process. In the lowest organisms, the simple protoplasmic mass divides, and each part retains all the physiological pro perties of the whole, and consequently constitutes a germ whence the whole body can be reproduced. In more ad vanced organisms, each of the multitude of cells into which the embryo cell is converted at first, probably retains all, or nearly all, the physiological capabilities of the whole, and is capable of serving as a reproductive gerrn ; but as division goes on, and many of the cells which result from division acquire special morphological and physiological properties, it seems not improbable that they, in proportion, lose their more general characters. In proportion, for example, as the tendency of a given cell to become a muscle cell or a car tilage cell is more marked and definite, it is readily con ceivable that its primitive capacity to reproduce the whole organism should be reduced, though it might not be alto gether abolished. If this view is well based, the power of reproducing the whole organism would be limited to those cells which had acquired no special tendencies, and conse quently had retained all the powers of the primitive cell in which the organism commenced its existence. The more extensively diffused such cells were, the more generally might multiplication by budding or fission take place ; the more localized, the more limited would be the parts of the organism in which such a process would take place. And even where such cells occurred, their development or non- development might be connected with conditions of nutri tion. It depends on the nutriment supplied to the female larva of a bee whether it shall become a neuter or a sexually perfect female ; and the sexual perfection of a large pro portion of the internal parasites is similarly dependent upon their food, and perhaps on other conditions, such as the temperature of the medium in which they live. Thus the gradual disappearance of agamogenesis in the higher animals would be related with that increasing specialization of function which is their essential characteristic ; and when it ceases to occur altogether, it may be supposed that no cells are left which retain unmodified the powers of the primi tive embryo cell. The organism is like a society in which every one is so engrossed by his special business that he has neither time nor inclination to marry. Even the female elements in the highest organisms, little as they differ to all appearance from undifferentiatcd cells, and though they are directly derived from epithelial cells which have undergone very little modification from the condition of blastomeres, are incapable of full develop ment unless they are subjected to the influence of the male element, which may, as Caspar Wolff suggested, be compared to a kind of nutriment. But it is a living nutri ment, in some respects comparable to that which would be supplied to an animal kept alive by transfusion, and its molecules transfer to the impregnated embryo cell all the special characters of the organism to which it belonged.

The tendency of the germ to reproduce the characters of its immediate parents, combined, in the case of sexual generation, with the tendency to reproduce the characters of the male, is the source of the singular phenomena of hereditary transmission. No structural modification is so. slight, and no functional peculiarity is so insignificant in either parent that it may not make its appearance in the offspring. But the transmission of parental peculiarities depends greatly upon the manner in which they have been acquired. Such as have arisen naturally, and have been hereditary through many antecedent generations, tend to appear in the progeny with great force ; while artificial modifications, such, for example, as result from mutilation, are rarely, if ever, transmitted. Circumcision through in numerable ancestral generations does not appear to have reduced that rite to a mere formality, as it should have done, if the abbreviated prepuce had become hereditary in the descendants of Abraham ; while modern lambs are born with long tails, notwithstanding the long-continued prac tice of cutting those of every generation short. And it remains to be seen whether the supposed hereditary trans mission of the habit of retrieving among dogs is really what it seems at first sight to be ; on the other side, Brown- Sequard s case of the transmission of artificially induced epilepsy in guinea-pigs is undoubtedly very weighty.

Although the germ always tends to reproduce, directly or indirectly, the organism from which it is derived, the result of its development differs somewhat from the parent. Usually the amount of variation is insignificant ; but it may be considerable, as in the so-called &quot; sports ; &quot; and such variations, whether useful or useless, may be trans mitted with great tenacity to the offspring of the subjects of them.

In many plants and animals which multiply both asexually and sexually, there is no definite relation between the agamogenetic and the gamogenetic phenomena. The organism may multiply asexually before, or after, or concurrently with, the occurrence of sexual generation. But ill a great many of the lower organisms, both animal and vegetable, the organism. (A) which results from the impregnated germ produces offspring only agamo- genetically. It thus gives rise to a series of independent organisms (B, B, B, . . .), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The pro cess thus described is &quot;what has been termed the &quot; alterna tion of generations &quot; under its simplest form, for example, as it is exhibited by the Salpce. In more complicated cases, the independent organisms which correspond with B may give rise agamogenetically to others (BJ, and these to others (B 2 ), and so on (e.g., Aphis). But, however long the series, a final term appears which develops sexual organs, and reproduces A. The &quot;alternation of genera tions &quot; is, therefore, in strictness, an alternation of asexual with sexual generation, in which the products of the one process differ from those of the other.