12 



OVUM. 



solid substance of organs or through mem- 

 branes ; and from the various stages of ad- 

 vancement of others already referred to, 

 seen in different parts of the same animal, 

 little doubt can prevail that they must have 

 done the same : but the aperture through 

 which they make their way, besides being in 

 most instances very minute, seems to close 

 very rapidly and completely after them. So 

 that the occurrence of entozoa in entirely 

 isolated cavities such as the aqueous cham- 

 ber of the eye, or in the parenchyma of solid 

 organs, does not now present to our minds 

 any valid objection to the view that in all in- 

 stances they are introduced from without ; 

 and it will be apparent, from the same con- 

 siderations, that even the occurrence of en- 

 tozoa in the foetus, of which there are un- 

 doubted instances, and to which great import- 

 ance has been attached as an argument in 

 favour of their spontaneous origin, may be 

 explained on the supposition of their ova, or 

 young, passing from the maternal parent, 

 through the blood-vessels of the umbilical 

 cord, as is known to happen with various 

 poisons. 



The whole history, then, of this remarkable 

 class of animals, as it is now known, tends to 

 support the general conclusion that they are 

 all capable in their complete state of sexual 

 reproduction, and that they gain the various 

 sites of their parasitic habitations by intro- 

 duction of their ova, or embryoes, or of more 

 advanced stages of their growth from without, 

 either directly into the open cavities, or more 

 indirectly, by piercing the coats of vessels, 

 membranes, &c., into the close cavities and 

 the parenchyma of solid organs.* 



A candid review of the whole evidence 

 on this question leads to the inevitable 

 conclusion, that, though all the difficulties 

 or doubts which surround it are by no 

 means completely removed, the hypothesis of 

 primary or spontaneous generation receives 

 little or no direct support from the accurate 

 observation of the mode of origin of those 

 animals which alone were supposed to afford 

 proofs of such a kind of production ; and that 

 this view must, therefore, on the strongest 

 grounds of analogy, be in the meantime aban- 

 doned, for that which attributes the origin 

 and reproduction of all organised beings to an 

 undeviating connection through ova or germs, 

 seeds or spores, between new individuals and 

 others of identical species which have pre- 

 viously existed. And if the present sonie- 



* As to the bearing of a knowledge of the habits 

 &c. of the Entozoa upon the question of their spon- 

 taneous origin, consult the able essay by Eschricht ; 

 " Inquiries concerning the Origin of Intestinal Worms 

 &c." in Eclin. New Phil. Journ. vol. xxxi. p. 314. 

 1841, the article on Parasites by V. Siebold, in E. 

 Wagner's Handwork der Physiol. ; E. Blanchard's 

 Ju'M'urches on the Structure &c. of Intestinal 

 Worms, in Ann. d. Sc. Nat. 1848 and 1849, parti- 

 cularly vol. vii. p. 121. Dujardin's systematic work, 

 Hist. Nat. des Ilelminthes, 1845. And in connec- 

 tion with this and the whole subject of spontaneous 

 generation, the Systematic Treatises on Physiology 

 of .Burdach, J. Miiller, Valentin, and Longet. 



what imperfect state of knowledge does not 

 permit us to affirm this absolutely, as the 

 result of direct observation, the exceptions 

 are so few and unimportant, that they may be 

 disregarded in the overwhelming evidence of 

 a positive character in favour of the opinion, 

 derived from analogy, that every organic 

 being, if not produced in actual union with 

 another, derives its origin from a germ or 

 some such connecting part that has proceeded 

 from a being of the same kind. 



If this be the present state of the argument 

 in respect to the hypothesis of the first origin 

 of organic beings, it need scarcely be added 

 that the opinion which has attributed the pro- 

 duction of various animals to conversion or 

 gradual transmutation out of other species or 

 genera, has still less of real to be adduced 

 in its support. In the long series of ages 

 in which authentic observations have been 

 made on animals, no such examples have 

 been ascertained, and there are no established 

 facts which give any substantial grounds for 

 believing that in the natural or wild state of 

 animals there is any departure from that un- 

 deviating succession of specific resemblance 

 between parent and offspring, which seems to 

 form one of the most constant of the laws 

 of organic nature with which we are ac- 

 quainted. 



3rd. Production of dissimilar individuals 

 among sexual animals by a non-sexual process : 

 so-called Alternate Generations. 



From the foregoing general views it ap- 

 pears that in all Vertebrated Animals, and 

 in by far the greater number of Invertebrated 

 animals, the process of permanent reproduc- 

 tion consists in the development of the new 

 being from the blastodermic mass formed by 

 a peculiar process of cytocenesis in the 

 fecundated ovum. But, as has already been 

 shortly stated, there are some varieties among 

 them in regard to the degree of directness 

 with which the product of development from 

 the ovum arrives at that state of maturity, or 

 sexual completeness, in which it is capable of 

 renewing the act of sexual generation. These 

 varieties may be classed as follows: 1st. The 

 product of the ovum, being single, attains by 

 a gradual process of development, when it 

 leaves the ovum at birth, to nearly the same 

 form and structure as its parents : this is 

 generally called Embruohgical Development. 

 2nd. The product of the ovum, being single, 

 is born or leaves the egg at an early period, 

 and while comparatively imperfect, or, as it is 

 called, in a larva state, and by one or more 

 successive changes of development of a marked 

 kind, afterwards reaches the specific or ty- 

 pical form : these changes are usually called 

 Metamorphoses. 3rd. The product of deve- 

 lopment from the ovum does not itself become 

 a complete animal, but gives rise, by a peculiar 

 mode of generation of a non-sexual character, 

 and therefore different from that by which 

 fecundated ova are formed, to a new body, or 

 to successive progenies of new bodies, one or 

 more of which ultimately attains to the specific 

 resemblance of the sexual parents by which 



