38 



OVUM. 



nutest state of division, and as differing from 

 ova in the absence of the germinal cell.* 

 Notwithstanding the attractive ingenuity of 

 these views, and the great weight which 

 all must be disposed to attach to the 

 statements of so accomplished an anatomist, 

 the theory they involve or expound, when 

 fully considered, does not appear entirely to 

 remove the veil from the mystery of alternate 

 generations, nor to afford that satisfactory ex- 

 planation or generalisation of its nature which 

 might be desired : for it is to be feared that 

 under the appellation of nucleated cells, as 

 applied to the structure of the lowest animals, 

 very various kinds of organised structures 

 have been confounded by authors, and we 

 are certainly very far from having had de- 

 termined, by actual observation, the nature or 

 source of the minutely granular masses from 

 which what has been called internal gemma- 

 tion proceeds ; and though it may be admitted 

 that most new structures take their origin in 

 masses of blastema, more or less cellular and 

 granular, the relation of these to the germ 

 masses of the ovum are far from being ascer- 

 tained. It may be granted, that in the case 

 of the first brood of Aphides formed in the 

 non-sexual way from the first individual im- 

 mediately developed in the ovum, a residuum 

 of germ-cells may have served as the original 

 blastema of their germs ; but when we consider 

 the inconceivably minute portions of this that 

 are to pass to the next and to the successive 

 generations up to nine or eleven, we seem to 

 have to deal rather with a theory of the 

 original pre-existence and " encasement of 

 germs" than with a matter that we can ever 

 hope to decide by observation : and in the 

 greater number of the other instances of 

 alternate generation, there is no possibility of 

 tracing the origin of the germs of the new 

 individuals formed by gemmation to the yolk 

 or germ- mass, of which they are regarded as 

 the included remains. 



Professor Owen has well remarked f , that 

 " in the Vertebrated, and in the higher Inver- 

 tebrated animals, only a single individual is 

 propagated from each impregnated ovum. 

 Organised beings might be divided into those 

 in which the ovum is uniparous, and those 

 in which it is multiparous. This is the first 

 and widest or most general distinction which 

 we have to consider in regard to generation, 

 and in proportion as we may recognise its 

 cause will be our insight into the true con- 

 dition on which Parthenogenesis depends." 

 But this distinction, notwithstanding its 

 acknowledged importance, does not carry us 

 any farther into the insight of the essential 

 conditions of the phenomena by the theory of 

 the residual germ-mass originating new germs; 

 for when the greater part of this mass is con- 

 verted into the textures and organs of the 

 embryo directly developed from the ovum, 

 there is still as great a difficulty as ever to 

 understand what circumstances should de- 

 termine a minute residuum, supposing it to 



* On Parthenogenesis, p. 38. 

 t Loc, cit. p. 62. 



exist, to form an entire new individual, or a 

 prodigious multitude of individuals, in place 

 of only an additional portion of texture, or 

 an additional organ, which might more nat- 

 urally be regarded as the correlative products 

 of their brethern germ-cells ; or why, in other 

 numerous instances, in which, to all appear- 

 ance, an equal quantity of residual germ-mass 

 exists, no such formation of new individuals 

 occurs. 



It appears equally fair to suppose that the 

 germs of the ova of all animals must have 

 originated within the ovaries of the female 

 parents from residual germ-cells included in 

 the blastema of these organs ; but no dis- 

 tinction has ever been established between 

 that blastema in its primitive state and that 

 of other organs of the body : and there does 

 not appear to be better reason for consider- 

 ing the germs of individuals formed by gem- 

 mation, as derived more directly from residual 

 and included germ-cells, than those of the 

 ovarian ova. 



I have not adopted the term Partheno- 

 genesis, as applied to the alternate generations, 

 because it implies that this kind of produc- 

 tion occurs in female animals. Now, although 

 the observations of Owen and V. Siebold 

 have shown a remarkable similarity to the 

 female structure in the case of the viviparous 

 Aphides, this is altogether wanting in the 

 other instances of alternate generations; and 

 this process is strictly non-sexual rather than 

 uni-sexual in its nature. Were it desirable to 

 change the name for this process, the term 

 " Metagenesis," suggested and sometimes 

 employed by Professor Owen, seems well 

 adapted to express exactly what occurs in 

 alternate generations, without calling for the 

 admission of any hypothesis beyond that of 

 the new production being an act of genera- 

 tion ; and it seems to me to be the most exact 

 translation in scientific terminology that can 

 be given of the German of Steenstrup in 

 " Generations- Wechsel." In a convenient 

 shape it is precisely descriptive of that change 

 of form by generation, or by production of 

 a new individual, which it has been my 

 object to show has been accurately dis- 

 tinguished in the general law of Steenstrup, 

 as different from a mere change of form by 

 growth or by metamorphosis in the same 

 individual. 



At the same time it is deserving of no- 

 tice, that, among the compound Polypina, 

 the continuous multiplication of individuals 

 proceeds to such an extent, and with such 

 remarkable regularity, as to give rise to com- 

 posite masses, often of very large size, the 

 general arrangement of which bears a con- 

 siderable resemblance to that of plants : and 

 we are led still more to institute a comparison 

 between these animals and plants by the cir- 

 cumstance of the similarity of their continuous 

 growth by the addition of new sets of polype 

 individuals to the extension of the leaves and 

 branches of a tree or ramified plant, and by 

 the correspondence of the occasional forma- 

 tion of sexual individuals on the polype stock 



