408 



ORGANS OF GENERATION. 



found rolling loosely within the body of the 

 larger animalcule, effecting their rotatory move- 

 ments by the agency of cilia of extreme minute- 

 ness, which under a good microscope are seen 

 to cover their external surface. The contained 

 globules having attained a sufficient maturity, 

 the parent volvox bursts, and thus by its own 

 destruction allows its progeny to escape from 

 their imprisonment. The multiplication of 

 these animalcules is effected with considerable 

 rapidity, and it not unfrequently happens that 

 even before the escape of the second generation 

 the gemmules of a third may be observed within 

 their bodies, which in like manner advancing 

 through similar stages of development will ter- 

 minate by their birth the existence of their 

 parent.* 



It would appear from the observations of 

 Professor Grant, that in the sponges, notwith- 

 standing their different form, the process of re- 

 production is entirely similar. In these curious 

 animals the gemmules are developed in the 

 substance of that living parenchyma which 

 coats their porous skeletons, and when mature 

 are expelled through the foecal orifices to com- 

 mence an independent existence. When sepa- 

 rated from the parent sponge, these gemmules, 

 like those of the volvox, are ciliated over a great 

 portion of their surface, and being thus endowed 

 with a power of locomotion, are enabled to 

 swim to a considerable distance in search of a 

 situation adapted to their future growth, until 

 having at length selected a permanent support, 

 they become attached, and developing within 

 themselves the spicular or horny skeleton pecu- 

 liar to their species, they gradually assume the 

 porous texture and particular character of the 

 sponge from which they were produced. 



But it is in the gelatinous Polypes that we 

 meet with the most perfect forms of gemmi- 

 ferous propagation : of this the Hydra viridis, 

 or fresh-water polype, affordsan interesting illus- 

 tration, and from the facility with which it may 

 be procured and examined by glasses of very 

 ordinary powers, it is well calculated to illus- 

 trate the mode of generation which we are at 

 present considering. The body of this simple 

 polype is transparent, and under the microscope 

 appears to be entirely made up of translucent 

 granules, without any trace of internal appara- 

 tus appropriated to reproduction. The gem- 

 mules by which it is propagated sprout from 

 some part of the surface, appearing first as mere 

 gelatinous excrescences, but gradually enlarging 

 they assume the form of their parent, acquiring 

 similar filamentary tentacles and a gastric 

 cavity of the same simple structure. As long 

 as the junction between the polype and its off- 

 spring continues, both seem to enjoy a commu- 

 nity of being, the food caught by the original one 

 being destined for the nourishment of both ; but 

 at length, the newly-formed animal having at- 

 tained a certain bulk, and become capable of 

 employing its own tentacles for the prehension 

 of food, detaches itself with an effort, and as- 



* [Some physiologists, however, refer the genera- 

 tion of this creature to the fissiparous mode. See 

 the succeeding article. ED.] 



sumes an independent existence. This mode 

 of multiplication is exceedingly rapid, a few 

 hours sufficing for the perfect developement of 

 the young creature; and not unfrequently even 

 before its separation, another gemmule may be 

 observed emerging from the newly-formed 

 polype, soon to exhibit the same form and ex- 

 ercise the same functions as the parent from 

 which it sprouts'? 



The propagation of some of the lithophytous 

 polypes resembles that of the hydra, the young 

 being produced from buds or gemmules, which 

 sprout from the living investment of their cal- 

 careous skeleton. Such are the Fungia, in 

 which the young are at first pedunculated, and 

 fixed to the laminse upon the upper surface of 

 the mass from which they spring ; in this state 

 they might readily be mistaken for solitary 

 Caryophyllia, but in time they separate from 

 the parent stock, and loosing the pedicle which 

 originally supported them, they assume the 

 form of their species. 



Oviferous generation. In the third, and by 

 far the most numerous division of the animal 

 kingdom, the young are derived from ova or 

 eggs, in which the germ of the future being is 

 evolved, and from which the young animals 

 escape in a more or less perfect state. 



It will be seen that the ovum which gives 

 birth to all the higher animals differs essentially 

 from the gemma furnished by the gemmiferous 

 classes ; in the gemmiferous type the bud or 

 offshoot of the parent appears by a kind of 

 vegetative evolution to assume the proportions 

 and functions of the original from which it 

 sprang. The ovum we would define as a 

 nidus, containing not only the germ of the 

 future animal, but a sufficient quantity of nu- 

 tritious matter, serving as a pabulum to the 

 embryo during its earliest state of existence, 

 and supplying the materials for its growth until 

 sufficiently mature to derive them from other 

 sources. We have already shewn that in the 

 fissiparous and gemmiferous animals there is 

 no necessity for any special generative appa- 

 ratus, but in the oviferous classes we find, for 

 the most part, a distinct system, more or less 

 complicated in structure, in which the repro- 

 ductive ova are developed and matured. It 

 must be confessed, however, that in the present 

 state of our knowledge upon this subject we 

 are not prepared to state how far the existence 

 of a generative system is exclusively confined 

 to the ovigerous type. We are well aware 

 that many authors describe generative canals 

 to exist in several of the polypiferous tribes, 

 although the reproductive germs produced from 

 them resemble in their ciliated organs of pro- 

 gression and mode of development the gem- 

 mules of less elaborately organized polypes ; 

 yet, on the other hand, as we have abundant 

 evidence to prove that such polypes as have the 

 ovigerous canals most distinctly formed, as the 

 Actiniae for instance, produce their young per- 

 fectly organized and evidently developed from 

 true ova, we are content, in the present state of 

 our knowledge upon this subject, to regard the 

 presence of generative canals as co-existent 

 with ovigerous generation, andv shall leave 



