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Observations on the Genetic Cycle in Organic Nature, and
particularly on the Relation between the different Forms of Alternation of Generations and the more Ordinary Modifications of the Reproductive Process. By GEORGE OGILVIE, M.D., Lecturer on the Institutes of Medicine in Marischal College and University, Aberdeen.*
§ 1. Origin of Organic Beings. The time is not yet out of mind when the doctrine of spontaneous generation was the great point of discussion in the physiology of reproduction. Now that this question has been set at rest by evidence as conclusive as any of a negative kind can well be, the attention of physiologists is chiefly directed to the relation between derivation in the ordinary way from two parents, and that other mode of origin from a single pre-existing form, of the prevalence of which among the lower species additional evidence is continually brought before us.
In this mode of origin, which has received various names from authors, such as Gemmation, Homogenesis, and Monogenesis, a portion of the body of the parent becomes the seat of a certain independent manifestation of vitality, whereby the plastic processes are so much intensified that, in
* Read before the British Association for the Advancement of Science, September 1859.
NEW SERIES - VOL. XI. Xo.1.-JAX. 1860.
the course of time, the part is converted into a distinct organism, capable of detachment from the parent, and fitted to maintain a separate existence. Such a detached gemma may be termed a free zooid, or phytoid. In the ordinary form of reproduction, again, that by the co-operation of the sexes, otherwise termed Heterogenesis, or Digenesis, a fusion takes place of two highly vitalized portions of the same or kindred organisms, and results in the formation of a fecundated germ, possessed henceforth of an independent vitality, endowed with a capacity for ultimately acquiring the structure characteristic of the species, and destined to be thrown on its own resources, by its extrusion from the protecting envelopes, as soon as its organisation is sufficiently advanced for this condition. In all but the very lowest forms of life, the conjugating algæ, a difference is observable between the two factors of embryonic life, which are recognised respectively as male and female, or as the spermatic and germinal elements.
§ 2. Relations of Ova and Gemmæ. It is strongly contended by some that there is such an incompatibility between these two modes of propagation, that, in proportion as any portion of the parenchyma of the parent is engaged in the one course, it is proportionally disabled for the other. This opinion is founded on these alleged peculiarities of the sexual elements in their mature condition; 1st, That singly they are not capable of any farther development, but very soon lose their vitality, and undergo decomposition ; and 2d, That they both differ very remarkably in appearance from all self-developing foci of vital action, and, in particular, that the germinal element of animals, or the unimpregnated ovum, though it may occasionally present a general resemblance to a gemma or bud, is always to be distinguished from it, by containing in its interior a peculiar nucleated cell, the germinal vesicle. But some, at least, of these peculiarities can no longer be maintained as constant characters. Quatrefages states that he has seen segmentation take place, independently of impregnation, in the ova of Hermella and Unio, though no farther development follows. In regard to the structural characters, again, even putting aside the evidence concerning the germs of the viviparous aphides as somewhat discordant, though, certainly, on the whole, in favour of the essential identity of ova and gemmæ, the observations of Mr Lubbock on the agamic ova of Daphnia, and those of Mr Smith of Kew, Professor Braun of Berlin, Radlkofer, and others, on the unimpregnated ovules of Coelobogyne, appear conclusive to establish that bodies elaborated side by side with the true germinal elements, and in some cases undistinguishable from them in appearance, may undergo development independently of impregnation ; while those of Dzierzon and Siebold on the hive-bee, go to show that the very same germs may undergo evolution either with or without impregnation, developing, in the former case, a female, and in the latter a male progeny.
. We can hardly, therefore, as it would seem, avoid adopting Professor Owen's conclusion, that there is no essential difference between an ovum and a gemma, and that the one may pass into the other by insensible gradations. We may assume, perhaps, that up to a certain point, the development of the new focus of vital action may go on all the same for a gemma or an ovum; but that towards the period of maturation the changes which take place in the latter to fit it for impregnation cause such a tension, as it were, of its vitality, as is incompatible with its continuance in the majority of cases, unless re-invigorated by the access of the spermatic element.
§ 3. Alternation of Generations.
Propagation by gemmation has been regarded as perpetuating the individual rather than the species, the successive zooids or phytoids preserving more completely than the progeny of embryonic origin the characters of the parent stock ; and it has been thought, too, that there is a tendency for the plastic power to wear out, in process of time, so that a recurrence of sexual generation at intervals is necessary to preserve the pristine vigour of the species.
However this may be, there is reason to believe that the