6 



OVUM. 



rapidly through fluids. From the first they 

 exhibit a minutely cellular and granular struc- 

 ture : but it does not appear that they are 

 originally formed from any single nucleated 

 cell : they appear rather from the first to lie a 

 congeries of cell progenies. They are desti- 

 tute of an external envelope ; hut, nevertheless, 

 it may often be difficult to distinguish between 

 them and true ova. 



The tendency to the multiplication of indi- 

 viduals by non-sexual reproduction is greatest 

 among those animals which are of the simplest 

 organisation, and more especially among those 

 in which the cellular structure predominates; 

 not that it is confined to them, nor that it 

 occurs in all animals so constituted, but that 

 it is much more frequent and complete in the 

 simplest animals of each class in which it has 

 been observed ; as if it were more liable to 

 occur in those species in which the process of 

 individual development had proceeded to the 

 least extent of advancement in the formation 

 of the living textures of their bodies. There 

 is accordingly a remarkable similarity in the 

 nature of the processes of non-sexual multi- 

 plication and ordinary growth in these very 

 simple animals ; and it is well known that the 

 same relation subsists between a low organi- 

 sation of animals, and their disposition or 

 power to repair individual parts of their 

 bodies lost by injury or accident.* 



1st. Of the Process of Reproduction in Pro- 

 tozoa, or animals in which the sexual distinction 

 hait not yet been discovered. 



Among the Protozoa reproduction takes 

 place in two modes, viz., 1st, by the process of 

 gemmation or fission, and, 2nd, by develop- 

 ment from separated gemmules or germs. For 

 an account of the first of these processes, the 

 reader is referred to the articles POLYGASTRIA 

 and PoRiFERA.f 



Among the Potygastria multiplication by 

 division is much more frequent than that by 

 gemmation. Tt consists in the fission or di- 

 vision of the whole unicellular body into two 

 nearly equal parts, each of which becomes, 

 when separate, a perfect animalcule like the 

 original one : in some the division is trans- 

 verse, in others longitudinal, and occasionally 

 it occurs in either of these modes in different 

 individuals of the same species. The nucleus 

 of the unicellular polygastria has been fre- 

 quently observed to undergo division previous 

 to the formation of the fissure, by which the 

 division of the external wall is completed. 

 a fact which has led some physiologists, as 

 Ehrenberg, M. Barry, and Owen, to attribute 

 to the nucleus an important influence in this 

 process of cleavage ; the first of these ob- 

 servers having even conceived the nucleus to 

 act the part of a male or fecundating organ. 



* See Mr. Paget's recent interesting lectures on 

 this subject, published in Medical Gazette, 1849. 



f A considerable number of the polygautrie infu- 

 soria described by Ehrenberg in his great work on 

 that class, are now very generally regarded as be- 

 longing to the vegetable rather than to the animal 

 kingdom, such as the families Closterina, Volvocina, 

 and Bacillaria. 



This latter view is not, however, adopted by 

 many of those who have made a study of 

 this class of animals. 



In some of the polygastria in which the 

 process of multiplication is either of a fissi- 

 parous or gemmiparous kind, as in Vorticella, 

 Uvella, and Polythalamous Rhizopoda, the 

 new individuals remain in connection, and are 

 associated together in branched pediculated 

 groups, in connected masses of a globular 

 form, or in regular spiral united series.* 



The Porifera, or sponges, appear to be re- 

 produced by a different kind of gemmation 

 from that now described in polygastria, viz., 

 by separate gemmules or small portions of the 

 substance of the sponge, which, soon after 

 having been detached from the main stock, are 

 moulded into a spherical form, and, being pro- 

 vided with cilia, move about in the water with 

 great vivacity for a considerable period. These 

 gemmules are thrown off in numbers propor- 

 tional in some measure to the activity of the 

 nutrition of the sponge, and therefore princi- 

 pally during the early part of summer. Towards 

 the approach of winter a different kind of re- 

 productive bodies is observed to be formed, 

 viz. small capsules containing globular germs, 

 which, after development within the capsule, 

 pass out of it and produce a new sponge for 

 every capsule or germ. These bodies have 

 been called ova, and certainly they bear very 

 great, resemblance to them ; but too little 

 is known of their nature and origin to enable 

 us to form an opinion whether they are to 

 be regarded as precisely of the same nature 

 as ova or not. In the mean time the}* may be 

 named the capsular germs.f 



But it appears that, among the polygastria, 

 and rhizopoda also, there are sometimes 

 formed, by a peculiar process not ascertained 

 to be of a sexual kind, minute reproductive 

 bodies of a cellular structure, which, if they are 

 not true ova, are at least substitutes for them.f 



* See an interesting paper by Dr. Carpenter on 

 the Genus Nnmmulina and other Foraminifera in 

 Quart. Journ. of Geol. Soc. Feb. I860. Some ju- 

 dicious and interesting remarks on this class of 

 animals, and on the relations and characters of the 

 Protozoa in general, are contained in a recent paper 

 by Mr. Huxley in the Annals of Natural History 

 (1851, vol. viii. p. 437.), in which he has described 

 a curious monocellular genus named Thalassicolla, 

 which occurs in masses, and forms spicula some- 

 what like a minute sponge. 



t See Laurent's elegant memoir, Eecherches sur 

 rilydre et 1'Eponge d'eau douce. 1842. 



J Allusion is not made here to the production of 

 granules by the difttuence of an infusorian animal- 

 cule erroneously taken by Ehrenberg for the depo- 

 sition of ova, but to a very different process. Du- 

 jardin, who pointed out this error (Hist. Nat. des 

 Infusoires, p. 101.), is of opinion that, besides the 

 processes of fission and gemmation, we know nothing 

 with certainty of the reproduction of infusoria ; but he 

 admits that it is possible that the minute bodies into 

 which an infusorian breaks up by diiiluence might 

 prove the germs of new individuals. Dr. Carpenter 

 has mentioned several instances of a kind similar to 

 those alluded to in the text, and has expressed the 

 opinion that something of the nature of sexual pro- 

 duction may yet be discovered to take place in these 

 animals (Prin. of Gen. and Comp. Physiol. p. tM!>, 

 and p. 917.). Observations of a similar kind are re- 



