[130] 



OVUM. 



substance of the sponge, and certain spherical 

 bodies enclosed by dense capsules, which are 

 produced towards winter, and which appear 

 to contain a number of germs, each of which 

 is capable of being developed into a Protean 

 animalcule, from which probably a sponge 

 may proceed.* But it may be doubted whe- 

 ther these last-mentioned capsules are true ova 

 or may not rather be of the nature of the gem- 

 mules, winter ova, or statoblasts of Professor 

 Allman; and it is important to notice that 

 Mr. Huxley has recently discovered in Te- 

 thya a different set of bodies, which contain 

 all the essential parts of true ova, viz. vitel- 

 line membrane, yolk, germinal vesicle, and 

 macula, and that these bodies, which are si- 

 tuated between the cortical and central sub- 

 stance, are imbedded in a mass of cells together 

 with spermatozoa.f Although the individual 

 living particles of the sponge closely resemble 

 simple ciliated infusoria, and the mass may, 

 therefore, be viewed as an aggregate of these 

 minute beings, yet its analogies with and 

 transitions towards the fungiform polypes are 

 so great, that we may expect ere long that 

 the phenomena of its reproduction may be 

 placed in a new and clearer aspect by the 

 continuation of the researches now noticed, 

 and by others of a similar kind. 



RECAPITULATION AND CONCLUSION. 



Having now stated in detail the principal 

 facts that have come under our knowledge 

 with regard to the form, structure, and mode 

 of origin of the ova of different animals, it may 

 be proper, in bringing this article to a close, to 

 endeavour shortly to deduce from these facts 

 the most general results to which they appear 

 to lead. These results, together with some re- 

 flections on our subject, may be stated under 

 the following heads, viz. 1. Definition of the 

 ovum, as related to its own structure, and its 

 history in connection with the reproduction of 

 the species. 2. Recapitulation of the most 

 general facts ascertained by the comparison of 

 the ova of different animals. 3. Morphology 

 of the ovum ; homology of its parts; and rela- 

 tion of the ovum to other organic structures. 

 4. Phenomena attendant on the maturation 

 of the ovum. 5. Relation of the ovum to 

 fecundation by the male sperm. 6. Immediate 

 effects of fecundation on the ovum ; and re- 

 lation of the ovum after fecundation to the 

 first commencement of the process of em- 

 bryonic development. 



1. Definition of the ovum, as related to its 

 own structure, and its history in connection 

 with the reproduction of the species. 



In the commencement of this article the 

 ovum was shortly defined as " the product of 

 parental sexual generation from which the 

 young of animals are developed (produced)." 

 This definition appears correct and sufficiently 

 comprehensive ; but should it appear desirable 

 to substitute for it a more precise description 

 of the characteristics of the animal ovum, the 



* See Carter in Annals of Nat. Hist. vol. iv. p. 89. 

 f See Mr. Huxley's paper in Annals of Nat. 

 Hist., 2nd series, vol." vii. p. 370. 



following may be proposed as applicable to 

 the ovum throughout the whole animal king- 

 dom, without involving any merely theore- 

 tical view of its structure and constitution, 

 viz. " the ovum may be shortly described 

 as a detached spheroidal mass of organised 

 substance, of variable size, enclosed by a 

 vesicular membrane, and containing in the 

 earlier periods of its existence an internal 

 cell or nucleus ; these parts, formed by the 

 female individual or organ of animals, are 

 capable, when fecundated by the male sperm 

 of the same species, of giving rise, by the series 

 of histogenetic and organogenetic changes 

 known under the general term of develop- 

 ment, to an embryo, from which either directly 

 or mediately the individuals of the animal 

 species to which the parents belong are re- 

 produced." 



We thus separate from the category of 

 true ova all those bodies of an apparently 

 reproductive kind which are not the direct 

 product of an act of sexual generation. To 

 such bodies, the nature of which is as yet 

 doubtful, and probably somewhat various, the 

 indefinite appellations of buds, bud-germs, 

 gemmae, spores, winter ova, ephippial ova, 

 statoblasts, &c., have been given according 

 to the circumstances in which they are se- 

 verally produced. 



In all animals, then, with the exception of 

 the Polygastric Infusoria and Rhizopoda, the 

 occurrence of sexual generation and the for- 

 mation of true ova are proved to be the 

 regular and constant means for the permanent 

 reproduction or maintenance of the species. 

 In the exceptional instances now mentioned, 

 and even in some others possessed of the 

 sexual distinction, the best known and most 

 common multiplication of individuals takes 

 place by a subdivision of the parent body, 

 either by fissiparous cleaving or by gemma- 

 tion; but in them also it can scarcely be 

 doubted that there are other means by which 

 the permanence of the species is maintained. 



All the most accurate recent investigations 

 lead to the conclusion that the production of 

 the young of all organised beings, even the 

 simplest of the Protozoa, does only occur by 

 direct connection through some organised 

 medium with other beings of a similar kind 

 or species. We are forced, therefore, to con- 

 clude that in the propagation or production 

 of these simple beings, in circumstances where 

 their more ordinary fissiparous or gemmi- 

 parous mode of multiplication cannot be ad- 

 mitted to have taken place, there must have 

 passed from the bodies of the progenitors 

 minute particles of organised substance (ca- 

 pable, as we know, of being suspended in the 

 atmosphere, and of resisting during a long 

 period many of those influences which gene- 

 rally prove inimical to animal development), 

 which particles, when brought into circum- 

 stances favourable to the progress of the vital 

 processes, undergo the cycle of changes ne- 

 cessary for the reproduction of beings similar 

 to those from which they sprang. If there is 

 any constant law which seems more certainly 



