434 



GENERATION. 



preparatory to their growth and conversion into 

 the fixed and immoveable Zoophyte. 



We may here recal to the recollection of the 

 reader that the different forms of non-sexual 

 reproduction which we have now attempted to 

 sketch are not confined respectively to par- 

 ticular classes of animals, for several of these 

 animals are reproduced in more than one 

 manner. 



The alleged instances of non-sexual propa- 

 gation occurring in animals higher in the scale 

 than those already mentioned are very doubt- 

 ful, and ought to be regarded either as founded 

 in imperfect knowledge of their reproductive 

 organs, or as rare exceptions to the general law 

 of their propagation by the sexual mode. * 



2. Sexual reproduction. The existence in 

 animals of generative organs of two kinds, and 

 the necessity of the co-operation of both these 

 organs in reproduction constitute the distinc- 

 tion of sex, or of male and female. In sexual 

 reproduction both kinds of organs produce a 

 substance essentially concerned in the process. 

 The product of the female organ, or ovarium, 

 as it is called, is the ovum or egg, a consistent 

 organised body of a regular and determinate 

 shape, in which the new animal is first formed 

 and resides during its early growth. A whitish 

 fluid is almost always the product of the male 

 organ or testicle, termed semen, or the semi- 

 nal fluid, from a belief formerly prevailing that 

 it constituted, like the seed, the greater part of 

 the new being. 



Nature of the ovum. The egg is naturally 

 produced by the female without the concur- 

 rence of the male, that is, the whole substance 

 is apparently formed by the female organ, but 



* In many of those instances in which female 

 animals have been supposed to give rise to produc- 

 tive ova, the males have at first escaped notice from 

 the smallness of their number or other causes ; and 

 with regard to others of the lower animals, it may 

 very reasonably be doubted whether the products 

 called ova have not been rather of the nature of 

 gemmae or sporules, such as those formed in the 

 Actinia and other animals naturally propagating in 

 the non-sexual manner. As in this predicament 

 may be mentioned, according to Burdach, Oxyuris, 

 Filaria, Ligula, Tricuspidaria, and others of the 

 Entozoa ; Serpula, Sabella, and other Tubicola ; 

 Cirrhopoda, and Mussels, and Scutibranchiata and 

 Cyclobranchiata, The Syngnathus was erroneously 

 regarded by Pallas, and the Perca Marina by 

 Cavolini, as propagating without sex. It is, how- 

 ever, probable that some animals provided with 

 both sexual organs, and which usually propagate 

 in the sexual mode, are occasionally reproduced 

 without the immediate concurrence of the male. 

 Thus the female Aphis, after being once impreg- 

 nated by the male, bears, for a certain portion of 

 the year, female young only, which are capable of 

 being reproduced for nine generations without any 

 of these female animals receiving any new influence 

 from the male. In the last of these generations 

 occurring in autumn, males also are produced which 

 impregnate the females destined to carry on the same 

 succession of generations during the next season. 

 According to some this extension of the fecun- 

 dating influence of the male through more than one 

 generation is not confined to the animals just men- 

 tioned ; but without doubt the instances in which 

 this has been supposed to be the case have been 

 greatly over-reckoned. 



the egg so formed is incapable of giving birth 

 to a new animal unless it receive a certain por- 

 tion of, or influence from, the seminal sub- 

 stance of the male. This addition of seminal 

 fluid to the egg makes no immediate percep- 

 tible alteration in its structure or appearance, 

 but awakens in it the power of reproduction, 

 fructifies or fecundates it by causing a physical 

 or vital change, the essential nature of which is 

 not fully understood. 



In the egg immediately after its fecundation, 

 none of the parts of the new animal are 

 visible. A certain time must elapse during 

 which the egg is exposed to certain favourable 

 influences of heat, air, &c. before the com- 

 mencement of those changes of development 

 and growth in which the formative process of 

 the new animal consists. The great mass of 

 the substance composing the egg consists of a 

 fluid, holding in suspension granules of animal, 

 albuminous, and oily matter. The form of the 

 egg is given by the external coverings, and 

 there is in every egg a determinate part or re- 

 gion, corresponding in all animals, at which 

 the small rudimentary parts of the embryo first 

 make their appearance. To this part of the 

 egg, which might be called its germ, the power 

 of independent life and reproduction appears 

 more immediately to belong; the granular fluid 

 serves but to afford nourishment to the young 

 being for a certain period. A gemma or spo- 

 rule, on the other hand, is generally held to 

 differ from the ovum in being homogeneous in 

 its structure, having no investing membranes, 

 and being entirely converted into the substance 

 of the new animal produced from it. In the 

 present state of our knowledge, however, the 

 distinction between an ovum and a sporule 

 must be admitted to be somewhat arbitrary. 



The position of the male and female genera- 

 tive organs upon the same or upon different 

 individuals, and the place or manner of the 

 development of the young animal from an egg, 

 are the two most prominent circumstances in 

 regard to which the forms of sexual reproduc- 

 tion differ from one another in various animals. 

 The principal processes in which sexual re- 

 production essentially consists, are, 1st, the 

 formation of an egg by the female organs ; 

 2nd, the secretion of the seminal fluid by the 

 male organ ; and 3rd, the union of the sexes, 

 and means by which the seminal fluid is ap- 

 plied to the egg so as to confer fecundity 

 upon it. 



Hermaphrodite generation. In some of the 

 lower tribes of animals belonging chiefly to 

 the Annelida, Acephala, and Gasteropoda, the 

 male and female sexual organs are placed on 

 one individual, an arrangement of the sexual 

 organs termed Hermaphrodite, and all the indi- 

 viduals belonging to one species are consequently 

 similarly formed In Insects, Crustacea, some 

 of the Mollusca, and all the Vertebrata, the dif- 

 ferent sexual organs are placed on two distinct 

 individuals, which are thus constituted respec- 

 tively male and female. In the greater number 

 of those animals in which the last-mentioned 

 arrangement exists, besides the sexual pecu- 

 liarities, there are in each certain general differ- 



