OVUM. 



[131] 



than others to result from all recent researches 

 into the history of organic nature, it is this 

 necessary connection by descent of one being 

 or set of beings from another. 



In all animals, with the exception of the 

 simplest tribes already referred to, the descent 

 from parent to offspring is through a product 

 formed and perfected only by the concurrence 

 of male and female organs ; but we are still 

 at a loss to determine whether the unseen 

 germinal bodies by which the Protozoa are 

 reproduced are of the same or of a different 

 nature. The structure of some of these ger- 

 minal bodies as described in the earlier part 

 of this article (p. 7., &c.), bears a very great 

 resemblance to that of true ova ; but yet the 

 sexual distinction of the parent animals has 

 not yet been discovered. The recent re- 

 searches of naturalists indeed show that our 

 whole knowledge of the history of the Pro- 

 tozoa may be considered as only in its infancy. 

 The discoveries as to the encysted stage of 

 existence among the Vorticellae and Gre- 

 garinae and others, the phenomena of conju- 

 gation observed in Gregarina and Actino- 

 phrys, the entire knowledge lately gained of 

 the form, structure, and habits of the Fora- 

 minifera, all point to important future dis- 

 coveries and modifications of our hitherto 

 crude and imperfect views of these tribes of 

 beings, and must make us refrain from at- 

 tempting at present to form any opinion or 

 even conjecture as to the modes of their re- 

 production ; while at the same time the recent 

 discoveries as to the existence of the sexual 

 distinction in the simplest forms of plants 

 encourage the hope that ere long the repro- 

 duction of the Protozoa may, in a similar 

 manner, be removed from the obscurity in 

 which it now lies hidden. It does not appear 

 necessary from these considerations that our 

 definition should make any direct reference 

 to animal bodies of the nature of which our 

 knowledge is still so imperfect. 



The result of development from a fecun- 

 dated ovum in all vertebrate and in a con- 

 siderable number of invertebrate animals, is 

 the formation of an embryo which, by a pro- 

 cess of progressive growth, arrives at matu- 

 rity, and assumes the form, structure, and 

 habits, either, as the case may be, of a her- 

 maphrodite animal, or of the parent of either 

 sex. In a certain number of these instances, 

 as in Batrachia, Insects, Crustacea, and others, 

 growth is not altogether continuously pro- 

 gressive, but is subject to one or more breaks 

 or changes as it were, which are marked by 

 some change in the mode of life, or some 

 difference in structure of the individual. To 

 such marked changes in the course of the 

 development or growth of an individual ani- 

 mal proceeding from a fecundated ovum, the 

 name of Metamorphosis is given. 



But from the facts narrated in the earlier 

 part of this article, it appears that in a cer- 

 tain number of the invertebrate animals, such 

 as those which have been referred to under 

 the heads of Echinodermata, Polypina, Aca- 

 lepha, Tunicate Mollusca, Trematode and 



Cestoid Entozoa, Annelida and Insecta, a 

 very different result may, either regularly and 

 constantly in some, or only occasionally in 

 others, attend the first development from the 

 fecundated ovum. To this modification of 

 the developing and reproductive process the 

 appellations of Alternate Generation or Meta- 

 genesis have been given, of which terms the 

 latter may perhaps be the most appropriate. 

 The phenomena which have been described 

 under this head are so very various, that it is 

 difficult, if not impossible, to give a short and 

 general statement of their nature. The dif- 

 ference between this and the better known 

 form of direct generation may, however, be 

 stated nearly as follows : In the Metagenetic 

 form of reproduction the individual formed 

 by the development of the fecundated ovum 

 is generally different in aspect, structure, and 

 mode of life from the parent or parents by 

 which the ova were produced ; this individual, 

 or zoi'd, though possessed, in many instances, 

 of an organisation and of powers which fit it 

 for the efficient performance of many of the 

 most important acts of independent animal 

 existence, is yet wanting in the attribute of 

 perfect animal maturity, viz., the sexual or- 

 gans and activity, and is consequently incapa- 

 ble by itself of an act of true generation, or, 

 in other words, of the formation of fecundated 

 ova, by which alone the species can be per- 

 manently reproduced. In such instances, 

 then, it is only by the formation from these 

 intermediate beings of others which are sexu- 

 ally perfect, that the generative act can be 

 repeated. There are two phenomena re- 

 quiring to be distinguished in connection with 

 the most common forms of this process ; the 

 one the frequent multiplication of the im- 

 perfect intermediate beings, or zoi'ds ; and the 

 other the production either directly or by a 

 succession of acts of development from the 

 intermediate beings of those which are sexu- 

 ally perfect, or which resume the form be- 

 longing to the parents from which the fecun- 

 dated ova were derived. It seems proper, 

 therefore, to distinguish between an act of 

 true sexual generation, and that by which 

 new beings are formed from the intermediate 

 individuals (or so-called nurses of Steen- 

 strup, or zoi'ds of other authors) ; the first 

 consisting invariably in development from a 

 fecundated ovum ; the second being probably 

 more analogous to a process of budding or 

 gemmation from a parent stock. It must be 

 confessed, however, that we have still much 

 to learn regarding the phenomena of this pro- 

 cess, before we can form any general notion 

 of its nature. The whole subject is replete 

 with the deepest interest not only in connec- 

 tion with the history of reproduction, but in 

 its influence, as stated in some parts of the 

 preceding article, on the whole range of zoo- 

 logical classification and distinction. Our 

 extended definition comprehends an allusion 

 to these phenomena. 



Lastly, the ovum may be considered as 

 having two phases or stages of existence ; 

 the one in connection only with the female 

 [K21 



