432 



GENERATION. 



scarcely entitled to reject the hypothesis of their 

 spontaneous generation, merely on the ground 

 that, in this respect, they do not agree with the 

 rest of the animal kingdom. Harvey even, 

 who established the proposition omne vivum ex 

 ovo, seems yet to have acknowledged the ne- 

 cessity of admitting some difference between 

 the more ordinary form of generation by means 

 of an egg and that which he called of the spon- 

 taneous kind.* 



In conclusion, we may remark, that while 

 we feel inclined to admit the existence of spon- 

 taneous generation among some species of cryp- 

 togamic plants, infusorial animalcules, and en- 

 tozoa, it must be held in recollection that many 

 of these productions, after their first origin, 

 propagate their species as parents, that the so- 

 called spontaneous kind of generation is to be 

 looked upon as no more than an exception to 

 the general law of reproduction, and that 

 therefore extreme caution is necessary in admit- 

 ting any organized body to be the product of 

 spontaneous generation upon the mere negative 

 evidence of the absence of its seeds or ova. 



II. SKETCH OF THE PRINCIPAL FORMS OF 

 THE REPRODUCTIVE FUNCTION IN DIFFE- 

 RENT ANIMALS. 



Before proceeding to detail the different steps 

 of Human Generation, which forms the more 

 immediate subject of consideration in this arti- 

 cle, we shall endeavour to present a short pre- 

 liminary sketch of the various forms which the 

 reproductive function assumes in different classes 

 of animals. 



Reproduction may be divided into non-sexual 

 and sexual, according as the whole process is 

 accomplished by one class of organs in a single 

 individual, or by the concurrence of two diffe- 

 rent kinds of organs placed either upon one or 

 upon two separate individuals of the same 

 species ; the first form occurring among the 

 simplest kinds of animals only, the second be- 

 longing to all the vertebrated and the higher 

 classes of invertebrated animals. 



\. Non-sexuul rtproduction. Of the non- 

 sexual mode of reproduction three principal 

 kinds may be distinguished, viz. first, by divi- 

 sion ; second, by attached buds ; and third, by 

 separated gemmae. 



Fissiparous generation. The most common 

 form of the fissiparous generation, as the first 

 of these varieties has been called, is met with 

 in some of the simpler Infusoria ; but it also 

 occurs occasionally in animals higher in the 

 scale. It consists essentially in the division of 

 the parent animal body into a certain number 

 of subordinate masses, each of which, being 

 endowed with independent life, becomes a new 

 individual similar to that of which it originally 

 formed a part. In some of the Infusoria in 

 which the process of subdivision has been mi- 

 nutely observed, fissures are seen to form in the 

 sides of the animal which is about to be repro- 

 duced ; these fissures gradually enlarge, and 

 meeting with one another, completely separate 



* Sre his Exercitations, as before quoted, 

 pp. 327 and 343. 



the parts. In one kind of fissiparous genera- 

 tion the parent body is split into irregularly 

 shaped masses, in some two in number, in dif- 

 ferent others, four, six, eight, or twelve, and in 

 one, the Gonium pectorale, into as many as six- 

 teen. Each of the subordinate masses, when 

 first separated from its fellow, has an irregular 

 shape, from which it gradually passes into the 

 form and size of its parent. 



In a second form of the fissiparous genera- 

 tion, the infusorial animal is divided into two 

 equal and symmetrical halves ; in some in- 

 stances in a longitudinal direction, as in Bac- 

 cillaria and some Vorticellse ; in others in a 

 transverse direction, as in Paramoecium, Cycli- 

 dium, and Trichoda. 



The propagation of the Volvox globator, a 

 remarkable infusorial animalcule, may perhaps 

 be considered as belonging to the first of the 

 above-mentioned varieties of fissiparous genera- 

 tion.* This animal consists of an external 

 vesicle of a lenticular shape, moving rapidly 

 through the water by means of cilia in a whirl- 

 ing manner. Within this outer vesicle there 

 are smaller ones of the same kind, in the inte- 

 rior of each of which still smaller ones may be 

 distinguished by the aid of a high magnifying 

 lens. The outer vesicle may be regarded as 

 the parent, and the inclosed vesicles as its 

 young, for in propagation the outer vesicle 

 bursts and is torn into shreds, while the inclosed 

 ones are set free, each of them to execute its 

 independent motions in the water, and in its 

 turn to burst, and thus propagate its like in 

 discharging those which it contains. 



A fissiparous kind of generation is not, how- 

 ever, confined to the Infusoria, but occurs also 

 in some of the Cestoidea and Annelida. The 

 most remarkable example is met with in the 

 Nais and Nereis. In the first of these genera, 

 a small portion separated from the tail becomes 

 the new animal. Before the actual separation 

 of this caudal portion, it is marked off from the 

 rest by a notch, and there are gradually formed 

 on its sides the joints, hairs, and other indica- 

 tions of the organs of the complete animal in 

 miniature. The notch enlarges, and the part at 

 last drops off capable of independent existence. 

 In the Nais, that part of the offspring by which 

 it is attached to the parent becomes the head, 

 and in this way, according to the singular 

 notion of Gruithuisen, who observed this sort 

 of reproduction with attention,t the tail of a 

 Nais may be considered as gifted with perpe- 

 tual life, since this part is extended into each 

 of the new descendants. J 



We may regard as somewhat analogous to 

 this kind of propagation the multiplication of 

 individuals by division, which happens occa- 

 sionally only or from accident in several of the 

 lower animals which are usually reproduced 



* Another view taken of the reproduction of the 

 Volvox globator is, that the young are formed in 

 the manner of internal buds. 



t Nov. Act. Nat. Curios, torn. xi. 



} The segments of the infusorial animalcule 

 that is propagating in the fissiparous mode are 

 united by the parts which afterwards become the 

 tails of the new individuals. 



