ORGANS OF GENERATION. 



407 



2nd. OemiuiparOM, in which the young 

 sprout as it were from the substance of the 



pan in. 



Ilrd. (h-i/iitnms, producing their ofi'spim.; 

 from ova cir gt -mis developed in special organs 

 adapted to their formation. 



Of these modes of reproduction the two first 

 are confined to the lowest or acrite division of 

 the animal kingdom, whilst the third or ovipa- 

 rous type is common to all other classes. 



Fisstparous generation is the simplest possi- 

 ble, and presupposes a corresponding simpli- 

 city of structure in the animals which propa- 

 gate in this manner. It is principally confined 

 to the Polygastric animalcules, most of which 

 are multiplied by the spontaneous separation of 

 an individual into two portions precisely resem- 

 bling each other, and capable of performing all 

 the functions which originally belonged to the 

 undivided creature. Some of the larger species 

 ofTrichoda are well calculated to exhibit to 

 the microscopic observer the steps by which 

 this process is accomplished : the animalcule., 

 prior to its division, is seen to become slightly 

 elongated, and a transparent line is gradually 

 distinguishable, indicating the course of the in- 

 tended fissure ; at each extremity of this line a 

 contraction of the body is speedily observable, 

 and the lateral indentations become deeper 

 and deeper, till at length a perfect separation 

 is effected. The direction in which this divi- 

 sion occurs is not always the same even in 

 the same species ; thus, instead of traversing 

 the shorter axis of the body it not unfrequently 

 assumes a longitudinal or oblique direction, 

 and from this cause it is not unusual to find the 

 newly divided creatures differing materially in 

 appearance from their adult or rather conjoined 

 form ; for in this process the old animalcules 

 literally become converted into young ones. 



In some of the more complex forms of Poly- 

 gastrioa the fissiparous mode of generation exhi- 

 bits modifications which are extremely curious. 

 In the beautiful ^urtlcellte, whose bell-shaped 

 bodies are supported on long and exquisitely 

 irritable stems, the division commences at the 

 brae dilated extremity of the animalcule, from 

 which point it gradually extends in a longitudi- 

 nal direction towards the insertion of the stem, 

 dividing the body into two equal portions, one 

 or both of which becoming speedily detached 

 from the pedicle, might easily in this state be 

 mistaken for creatures of a different genus, and 

 have in fact been described as such by many 

 authors. The new animalcule, when thus 

 deprived of its pedicle, is seen to be fur- 

 nished with cilia at the opposite extremity to 

 that on which they were previously found, while 

 from the other end, originally the mouth, a new 

 foot-stalk becomes gradually developed, and the 

 creature assumes the shape proper to its species. 

 If one of the bells remain attached to the pedi- 

 cle, it continues to perform the same movements 

 as before the separation of the new animalcule ; 

 but if both become detached, the foot-stalk 

 perishes. 



In the strangely compound symmetrical bo- 

 dies of Gotuum a provision for separation 

 appears to be made in ihe detached portions of 



which each perfect animal is apparently com- 

 posed. The body of (inruinn ptettntt con- 

 sists of sixteen minute transparent globes of 

 unequal size, arranged in tiie same plane. 

 This beautiful animalcule is ptopaguted by 

 a separation of its integrant spherules, the 

 creature dividing into four portions precisely 

 similar to each other, and composed individually 

 of one of the central globules united to three 

 of the smaller marginal ones ; and no sooner is 

 the division accomplished than the component 

 globes of each portion increasing in number, 

 the new animalcules assume the dimensions 

 and appearance of that of which they originally 

 formed parts. 



In the Gonium pulvinutum the fissiparous 

 mode of generation gives origin to a still more 

 numerous progeny. The young animalcule is 

 a minute, flat, diaphanous and quadrangular 

 membrane, which swims through the fluid in 

 which it is found by movements sufficiently in- 

 dicative of its animal nature : as it enlarges, the 

 surface is seen to become marked by two series of 

 parallel lines which cross each other at right an- 

 gles and divide the creature into smaller squares, 

 which ultimately separate and become di-tinct 

 representations of the original animalcule. 



Some of the Nematoid worms, as the Nais, 

 are likewise said to propagate by spontaneous 

 division. 



d tmmipurous generation. This mode of re- 

 production, like the fissiparous, is confined to 

 the lowest tribes of animal existence, and the 

 creatures which propagate in this manner are 

 unprovided with any apparatus specially appro- 

 priated to generation. The young appear as 

 gemmae or buds, which at certain periods sprout 

 from the homogeneous parenchyma which com- 

 poses the body of the parent, and these buds gra- 

 dually assuming the form of the original by a 

 kind of vegetative growth, become in a short 

 time capable of an independent existence. The 

 gemmiparous type of the generative function 

 is met with through a wider range of the animal 

 kingdom than the last, existing under modified 

 forms in many species of Polygastric Infusoria, 

 and of Polyps, as well as in Sponges, the Cys- 

 tiform Entozoa, and probably in some Aeale- 

 phse. 



It is in the Cystoid Entozoa that we find it in 

 its simplest form. In the Cysticercus and like- 

 wise in the Ccenurus, the transparent membra- 

 nous bag of which the animal consists is filled 

 with a glairy fluid, in which occasionally young 

 hydatids are seen floating about. These young 

 Cysticerci in the earliest period of their forma- 

 tion are seen to pullulate from the parietes of 

 the parent sac, and gradually enlarging they 

 ultimately separate from their connexions, be- 

 coming detached and perfect animals. 



Many of the Polygastrica are multiplied by 

 a similar process, of which the Volvox globular 

 may serve as an illustration. This beautiful ani- 

 malcule is a minute diaphanous globe, which 

 under the microscope is generally seen to con- 

 tain a variable number of smaller globules, which 

 are the young : these, when first discoverable, 

 arc attached to the inner surface of the parent, 

 but speedily detaching themselves they are 



