OVUM. 



spermatic matter derived from detached 

 chains which are frequently floating near 

 them. To this result, no doubt, the currents 

 of water in the respiratory cavity will materi- 

 ally contribute. 



Since the publication of the first part of 

 this article, the general aspect also of the 

 questions involved in the facts referred to has 

 undergone revision by several authors. The 

 whole doctrine of alternate generation has, it 

 appears, been called in question by Reichert, in 

 a programme entitled Unisexual Reproduc- 

 tion*, which I have not had an opportunity 

 of seeing. In his recent very able and in- 

 structive treatise on Reproduction in the 

 Handworterbuch der Physiologic, Rud. 

 Leuckartf, though he retains the name of alter- 

 nate generation as designating the phenomena 

 referred to, gives the doctrine little place, and 

 seems still inclined to regard this mode of pro- 

 duction rather as a peculiar modification of 

 growth than as a true generation. He is per- 

 haps right in his remark^:, that physiologists 

 have been too much disposed to separate the 

 phenomena of alternate generation from other 

 forms of production of a non-sexual kind, of 

 which he considers them as only a variety ; the 

 alternation of different forms being, according 

 to him, only a subordinate and not an essential 

 phenomenon. In his recent very interesting 

 work on Animal Morphology, Victor CarusJ 

 has allowed more importance to the views of 

 Steenstrup, adopting at the same time Owen's 

 term Metagenesis, as most suitable for the 

 designation of this kind of production. This 

 author appears to me to have made the 

 nearest approach to a correct appreciation of 

 the nature of this process in its relation to the 

 whole phenomena of animal reproduction as at 

 present known. 



Reviewing therefore finally the whole of the 

 facts and opinions on this subject, I am in- 

 clined to adhere to the views expressed in a 

 previous part of this article, that the pheno- 

 mena of alternate generation or metagenesis 

 ought to be grouped together, and to retain 

 their place as constituting one of the general 

 forms of reproduction among animals ; con- 

 sisting, as they constantly do in a certain 

 number of animals, in the combination, alterna- 

 tion, or succession of the sexual and non-sexual 

 production of individuals all proceeding origin- 

 ally from the development of one ovum. At 

 the same time it is to be noted, that this mode 

 of production does not exist in all the species 

 or genera of those tribes of the lower animals 

 among which it has been observed to occur, 

 and that in some it passes by gradual transi- 

 tions into other forms of the generative pro- 



* Die Monogene Fortpflanzung. Dorpat, 1852. 



t Article Zeugung, in the 5th and 6th parts of 

 vol. iv. of Wagner's Handworterbuch, 1853. 



J Loc. cit. p. 979. 



System der Thierischen Morphologic. Leipzig, 

 1853. 



cess. It is not on that account the same as 

 them. Metamorphosis and Metagenesis, also, 

 as V. Carus remarks, may be combined, but 

 they are different. " Larvae, the subjects of 

 metamorphosis, arrive at the state of perfec- 

 tion by throwing off provisional structures 

 which belong to their larval condition, but 

 nurses, the subjects of metagenesis, are them- 

 selves entirely provisional structures." 



Generation, therefore, or the production of 

 new individuals belonging to a species, may be 

 either of the sexual or of the non-sexual kind. 

 It is only in the Protozoa that the distinction of 

 sex has not yet been discovered. In all other 

 animals the production of new individuals of 

 the species is the result of the development of 

 ova fecundated by spermatic matter. In all 

 the Vertebrate animals and in the majority of 

 the Invertebrate, the development of the 

 ovum gives rise to a single new individual ; in 

 most of them by a continuous process of 

 formative growth, in some by successive 

 stages, or by metamorphosis. In a few Inver- 

 tebrate animals the development of the ovum 

 gives rise directly to more than one individual 

 by what may be named primary division of the 

 ovum. In a considerable number of Inverte- 

 brate animals the production of new indivi- 

 duals sexually perfect is not immediate from 

 the fecundated ovum, but secondary or in- 

 termediate by non-sexual formation from a 

 preparing stock which is the product of de- 

 velopment from the ovum. In such animals 

 an alternation of sexual and non-sexual for- 

 mation of individuals is necessary for the 

 completion of the act of generation or the 

 reproduction of the species. In a few of 

 these animals only one new individual is 

 formed ; but in by far the greater number the 

 product is multiple, thus leading to a great 

 increase in the number of individuals, either 

 in the distinct or in the aggregate form, which 

 have all derived their origin from a single ovum. 

 It is to be observed however, that in some in- 

 stances the alternating generation may co-exist 

 either with the non-sexual mode of multipli- 

 cation or with direct sexual reproduction. 



In conformity with these views of the rela- 

 tion of the product of generation to the ovum 

 and its progenitors, there might thus be esta- 

 blished the following modes of reproduction 

 among animals viz. 



I. MONOGENESIS, reproduction without 

 known sexual distinction. 



II. DIGENESIS, bisexual reproduction. 



III. METAGENESIS, alternation of sexual 

 and non-sexual reproduction. 



In each of these main forms of the repro- 

 ductive process the following varieties might 

 also be distinguished viz. 



1. With single progeny. 



2. With multiple progeny. 



3. By consecutive development. 



4. By metamorphic or interrupted develop- 

 ment. 



