565 



REPRODUCTION IN PLANTS AND ANIMALS. 



REPRODUCTION IN PLANTS AND ANIMALS. 



566 



water, oil of sweet almonds, iodine, sulphuric acid, and alkalies. The 

 general contents of the grain of pollen are starch, and this either 

 proceeds unchanged downwards through the pollen-tube, or else passes 

 along after being changed by a chemical vital process into a trans- 

 parent and colourless fluid, which becomes gradually more and more 

 opaque ; and is coagulable by the application of alcohol ; out of this, 

 by an organising process, the cells are produced which fill the end of 

 the pollen-tube, extending in Orchis Morio far beyond the ovule, and 

 thus forming the parenchyma of the embryo. 3, The identity of the 

 embryo and the pollen-tube is further supported by the fact, that in 

 such plants as bear several embry oes, there is always precisely the same 

 number of pollen-tubes present as we find embryoes developed. 



These views of Schleiden, with his conclusion that the pollen-tube 

 should be regarded rather as the representative of the female than of 

 the male in the animnl kingdom, have been adopted by Wydler of 

 Berne and others. 



On the other hand, observations were made by Messrs. Mirbel and 

 Spach, on another class of plants, in which they did not meet with the 

 structure described by Schleiden, and consequently they object to the 

 general application of his conclusions. They examined the develop- 

 ment of the ovule of the Zea Mays (Common Maize). In this plant 

 there is no true embryo-sac, but they found the commencement of 

 the embryo, which they call the primary utricle, and which Schleiden 

 described as the result of an involution of the sac, existing in the 

 cavity of the nucleus. In this plant also, only one, and not two 

 membranes, as described by Schleiden, existed in the embryo. They 

 also found in other plants the primary utricle existing in the interior 

 of the embryo-sac, and at a period anterior to the act of impregna- 

 tion. They therefore conclude that the pollen-tube does not become 

 the embryo, and that no involution of the embryo-sac takes place. The 

 ir conclusions are probably as much too general as those of Schleiden. 



Mr. Griffiths, in a paper published in the ' Linn&an Transactions,' 

 gives the result of a long series of investigations on the development 

 of the ovulum hi the genera Santalum, Ogyris, Loranthta, and Viscum. 

 From his observations on these plants, which differ from those investi- 

 gated by Schleiden, and Mirbel and Spuch, he has arrived at conclu- 

 sions somewhat different from those of any of these observers, and 

 he carefully refrains from drawing an inference from the facts which 

 he has observed that would apply to the whole vegetable kingdom. 

 " The first process," he says, " in the development of the seed subse- 

 quently to the penetration or application of the boyau (the pollen- 

 tube) to the embryo-sac would, in Santalum, Osyris, Loranthta, and 

 Vitcum, appear to consist of the formation of cellular tissue. This 

 may be applied, I believe, to most if not to all instances. This 

 cellular tissue appears to have two different origins ; one, and this is 

 the earliest in development, being perhaps referable to the embryo- 

 sac, while the other appears directly referable to the anterior ends of 

 the pollen-tubes." Thus far he agrees with Schleiden, that the pollen- 

 tube penetrates into the embryonal sac, and that the embryo is 

 derived from its intruded extremity, his observations on Santalum 

 and Loranthuf confirming this fact, whilst OtyHs is an exception con- 

 firmative of the rule. "But none of my observations," says Mr. 

 Griffiths, " have tended to confirm Schleiden's idea of the inflection 

 of the embryo-sac before the pollen-tube; and it appears to me 

 sufficiently obvious, that if such were the case the cylindrical bag 

 (the primary utricle of Mirbel), constituting the embryo in its first 

 stage of development, would consist of three membranes or layers, 

 namely, the first, or outer, of the ordinary and uninflected membrane 

 of the sac ; the second, of its inflected portion ; the third, that of the 

 pollen-tube itself." He also expresses his conviction that the 

 primordial or primary utricle of Messrs. Mirbel and Spach is the sac 

 of the embryo, which no doubt often and perhaps generally exists 

 before fecundation. 



Dr. Giraud, has published a paper in tho same volume of the 

 'Lbmtaan Transactions.' He made a series of observations upon the 

 ovulum of the Troposolum majiw. He concludes from his observations 

 on the Tropaolum majut, " that in this plant the primary utricle and 

 the future embryo never have any structural connection with the 

 extremity of the pollen-tube at their first origin, or at any subsequent 

 period of their development, as is sufficiently obvious from the fact 

 that tho pollen-tube is never brought into contact with the embryo- 

 sac. As the primary utricle makes its appearance before impregna- 

 tion has occured, it cannot be possible that the organ has ever formed 

 the extremity of the pollen-tube, as is believed by Schleiden and 

 Wydler. Moreover, as the primary utricle takes its origin wholly 

 within the embryo-sac, and at the earliest period of its formation is 

 not in contact with that membrane, it cannot have been formed by the 

 pollen-tube pressing before it a fold of tho embryo-sac in its passage 

 into the cavity of that structure, as Schleiden has maintained." 



In the 'Annals of Natural History,' 1852, Professor Henfrey has 

 published a paper on the Reproduction of the higher Cryptogamia 

 and P/ianerogamia, in which he states that he has not been able to 

 observe the penetration of the pollen-tube into the embryo-sac. 



We now pass to the consideration of the function of Reproduction 

 amongst Animals. General Reproduction occurs in many of the lower 

 animals in the same manner as plants. There is a common reproduc- 

 tion of destroyed tissues which frequently extends to the production 



of an entire limb. This is seen amongst the Jltadiata, especially the 

 Echinodermata, also amongst the Articulata. The highest families of 

 animals in which this kind of reproduction occurs regularly are the 

 Reptiles, in which instances are recorded of legs and tails being 

 renewed. Occasional instances occur in which the limbs of higher 

 animals are reproduced. The case of a Thrush, in which such renewal 

 had taken place hi a leg, was brought before the British Association 

 meeting at Hull. A case is also related in which an abnormal finger 

 in a human being having been removed, it was again reproduced 

 almost entire. 



Reproduction by division into two, or by gemmation, the Fissipar- 

 ous and Gemmiparous methods of Reproduction, occur to a very con- 

 siderable extent among the lower animals. These modes of reproduc- 

 tion do not essentially differ, and both occur in the same families of 

 animals. [HYDHA ; ACALEPH* ; POLYZOA.] The individuals which 

 are thus produced by fission or by gemmation are called Zooids. This 

 process occurs in unicellular as well as multicellular plants and 

 animals, and the single cells produced by the division of the Desmidiece, 

 the Dialomacece, and the Vorticelleiz, are as much entitled to the term 

 Zooids as the more complicated forms of the Acaleplus. 



The true generative act is performed in animals in the same manner 

 as plants. In order to the production of the new individual it is 

 necessary that there should be a union of germ-cells on the one hand 

 with sperm-cells on the other. We shall not here attempt to describe 

 the various forms of organs in the animal kingdom in which these 

 sperm-cells and germ-cells occur. They are described in considerable 

 detail hi this work under the head of the families, and sometimes of 

 the genera and species of the various animals described. We shall 

 however describe generally the nature of these cells. The germ-cells 

 and sperm-cells in animals are usually produced from tissues and 

 organs that are structurally different, but as in plants these organs may 

 be placed on different individuals, or on the same. When the two sets 

 of cells are found on the same individual, or zooid, they are said to be 

 Hermaphrodite ; but if these cells are found on different individuals 

 they are said to be Monosexual. The term hermaphrodite is also 

 applied to plants ; but when their sperm-cells and germ-cells are placed 

 on different flowers, as happens sometimes in the Phanerogamia, they 

 are called Monoecious and Dioecious. 



The sperm-cells in the animal kingdom assume a more definite form 

 than those of the vegetable kingdom. In the higher Cryptogamia, 

 where they assume the form of self-moving filaments, they most closely 

 resemble those of the animal kingdom. These filaments are formed 

 in the interior of cells, from which they escape by bursting. They 

 usually present an elongated filamentous appearance, with a slight 

 dilatation at one extremity. At one time they were regarded as a kind 

 of animalcule, and called ' spermatic animalcules,' and were supposed 

 to have an interior organisation. This is not the case, and they have 

 no more claim to be regarded as animalcules than moveable blood- 

 discs, or ciliated epithelium-scales. The movements performed by 

 these bodies are in many instances due to the presence of cilia, which 

 are found upon their surface. The movements of such filaments 

 would vary according to the disposition of the cilia. In other cases 

 the movement seems due to molecular activity. The object is very 

 obviously to bring the spermatozoon, as these spermatic filaments have 

 been called, into contact with the germ-cell. These movements soon 

 cease after the filaments have been removed from the matrix ; n 

 which they have been formed. Some agents rapidly destroy these 

 movements, whilst others renew them after they have apparently 

 ceased. This subject has been recently investigated by Kolliker, and 

 the results which he has arrived at in regard to the movements 

 observed in the spermatic filaments of Mammalia, are embraced in the 

 following propositions : 



1. In the spermatic fluid, taken from the epidermis and vas defereas, 

 motile spermatic filaments exist in very great abundance. 



2. In water and aqueous solutions of all innocuous indifferent sub- 

 stances and salts, the motion of the filaments ceases, and they form 

 loops. 



3. These filaments, thus furnished with loops, are not dead, as has 

 hitherto been generally believed ; for, on the contrary, they revive 

 completely upon the subsequent addition of concentrated solutions 

 of innocuous indifferent substances (sugar, albumen, urea), and of 

 salts. 



4. In all animal fluids, when considerably concentrated, or highly 

 saline, which are not too acrid nor too alkaline, nor too viscid, the 

 motions of the spermatic filaments are unimpaired ; this is the case, 

 for instance, in blood, lymph, alkaline or neutral urine, alkaline milk, 

 thin mucus, thick bile, the vitreous humour but not in saliva, acid 

 or strongly ammoniacal urine, acid milk or mucus, the gastric juice, 

 thin bile, and thick mucus. When the proper degree of concentration 

 of the latter fluids is successfully attained, and then: reaction la 

 rendered neutral, they are innocuous. 



5. In all solutions of indifferent organic substances moderately con- 

 centrated, the filaments move with perfect facility thus in all kinds 

 of syrup, hi albumen, urea, glycerin, salicin, amygdalin. More con- 

 centrated solutions of? these substances cause the motion to cease, 

 but it is restored upon their subsequent dilution with water. Too 

 dilute solutions act in the same way as water (vide 2 and 3). 



0. Certain solutions, as they are termed, of indifferent organic 



