OVUM. 



13 



the ova were produced. This is the " Alter- 

 nating Generation" of Steenstrup, or what 

 we might with Mr. Owen, in contrast to Me- 

 tamorphosis, call a process of Metagenesis* ; 

 and of which the single and multiple varieties 

 might be distinguished according as the inter- 

 mediate progeny consists of one or of suc- 

 cessive new productions. 



In the two first and best known forms 

 of sexual generation, the term Development 

 has been usually given to a gradual process 

 of changing and advancing growth by which 

 the new animal is formed out of the ovum, 

 till the period when it leaves it, or is said 

 to be born ; and the term Metamorphosis has 

 been generally applied to certain more marked 

 and sudden changes of growth, apparently 

 depending on the circumstance of the embryo 

 or young animal having left the ovum, or 

 having been born, at an early period in a com- 

 paratively incomplete state of growth. But 

 in establishing such a distinction between 

 these terms, it is not meant to be affirmed 

 that the changes which a young animal sub- 

 ject to metamorphosis undergoes are indi- 

 vidually or on the whole greater than those 

 which occur in an animal which attains to its 

 full growth by a process of development ; but 

 merely that the one series of changes is less 

 gradual than the other; and that the more 

 marked changes which accompany metamor- 

 phoses are related to certain conditions neces- 

 sary to enable the animal which is born at an 

 early period immediately to perform those acts 

 which belong to its independent existence. 

 It would indeed not be difficult to show that 

 the changes which a mammal or a bird under- 

 goes during its viviparous or oviparous de- 

 velopment, are quite as remarkable and com- 

 plete as those which occur in the change of a 

 Batrachian reptile from its aquatic to that of 

 its air-breathing condition, or of an insect 

 from its larva to its complete form. 



In both of these instances one individual 

 only is developed from the ovum, and that 

 individual itself at last reaches sexual com- 

 pleteness, and as being well understood they 

 need not be longer dwelt upon here. But in 

 the varieties of the reproductive process which 

 are now to be more particularly noticed, 

 the individual that proceeds directly from 

 the ovum does not itself pass through the 

 whole series of changes which are necessary 

 to bring it to the form of the fully developed 

 animal; but before it possesses any sexual 

 organs, or has attained to sexual maturity, it 

 produces from a minute germ formed in its 

 body by a non-sexual process, a new indi- 

 vidual, or a succession of individuals, the last 

 of which only attains to the specific resem- 

 blance of the parents, and acquiring sexual 

 organs propagates the species by means of 

 ova. This is the modification of the repro- 

 ductive process already termed Metagenesis, 

 and which has received so much attention 

 under the name of ** Alternate Generations" 

 since the publication of Steenstrup's cele- 



* Adopting a terra which has been used by Mr. 

 Owen in his Lectures. Med. Times, vol. xx. 1849. 



brated treatise under the title of " Generations- 

 Wechsel" in 1842* 



No examples of this peculiar modification 

 of the reproductive process have been known 

 to occur in the Vertebrata, and with one ex- 

 ception they are confined to the lower and 

 simpler of the classes of Invertebrated animals. 

 They are not, however, entirely confined to the 

 very lowest classes of these animals as dis- 

 tinguished by the Zoologist, but rather to the 

 simpler and less developed members of each 

 of the several classes in which instances of 

 them have been hitherto observed. 



The essential nature of this form of repro- 

 duction consists, then, in the development 

 from the ovum of an individual which is dis- 

 similar from the parent or parents producing 

 the ovum, and in the succeeding production 

 from that individual, by a non-sexual process, 

 of a progeny of one or more, or a succession 

 of individuals, of which the last of the series 

 resumes the parental form. While in animals, 

 therefore, reproduced by the ordinary form of 

 generation, the species is composed of entirely 

 similar individuals, or of individuals differing 

 only in sex ; in those animals which are sub- 

 ject to the alternate or intermediate genera- 

 tion, the species includes a variety of indi- 

 viduals usually of dissimilar form ; of which 

 some are without sex, and others are com- 

 plete as regards the development of sexual 

 organs. 



It has appeared to some authors that the 

 phenomena in question are to be regarded as 

 no more than peculiar modifications of the 

 processes of development or metamorphosis, 

 of such a nature that the product of the ovum 

 becomes multiple instead of, as is more usual, 

 remaining in its single individuality. But to 

 admit the correctness of this view, it would 

 be necessary to employ these terms in a sense 

 widely different from that commonly given to 

 them ; and, indeed, to modify the ideas of 

 these processes of embryological development 

 in a greater degree than seems warranted by 

 what is at present known of their nature. 



The name of larva is usually given to the 

 imperfectly developed animal that is born or 

 leaves the egg at a comparatively early period, 

 and fitted for independent existence in that 

 state ; and in the changes of metamorphosis 

 by which that larva attains to the complete 

 specific form, great as these changes may in 

 some instances be, we recognise that it is the 

 individual produced from the ovum which 

 itself undergoes these changes ; whereas in 

 the various kinds of alternate generation, it is 

 always by the formation of an entirely new 

 individual, arising from a minute germ con- 

 nected with the first, but to be distinguished 

 from its parts, and without a sexual process, 

 that the species is at last completed. The 

 new individual may be single or there may be 

 a multitude of them ; they may remain con- 

 nected with the one producing them or they 



* A work which appeared originally in the Danish 

 language, and in German in 1842, and of which an 

 excellent translation into English has been pub- 

 lished by the Ray Society in 1845. 



