SI9 



GELDER ROSE. 



GENERATIONS, ALTERNATION OF. 



950 



an actual cellular form like the latter. There is however a resem- 

 blance between the two in several points, especially in relation to 

 the large proportions in which they both exist, and to the several 

 functions which they perform. 



Gelatigenous substance is so widely diffused over the body that it 

 would exhibit the entire shape of the principal organs, even if all other 

 constituents were separated. It constitutes the skin, the serous mem- 

 branes, the cellular sheaths of the muscles, the organic portion of bone, 

 and many other substances. It is insoluble in cold water ; acetic acid 

 renders it transparent and bulky; tannic acid renders it solid, and 

 prevents its putrefaction ; and when boiled it forms a jelly. It is in 

 consequence of the last property that it has received the name of 

 Gelatin. The gelatigenous substance (as in skin, areolar tissue, serous 

 membranes, &c.) is insoluble in cold water, and on boiling is merely 

 physically and not chemically altered. In the process of boiling 

 nothing is taken up and nothing separated ; the alteration being similar 

 to that undergone by starch when heated in water. 



The composition of Gelatin is represented by the formula 



C 13 H 10 N, 5 



whether obtained from hartshorn, from isinglass, or from silk. Both 

 boiled and unboiled cellular tissue (after its conversion into glue) 

 combine with tannic acid, and produce compounds which are insoluble 

 in water and resist putrefaction ; hence the power of all medicines 

 containing this substance to heighten the tone of the system. The 

 protein-compounds in a similar manner form hard and coherent com- 

 pounds with tannic acid. Peruvian and willow bark, catechu, and 

 many other astringent medicines produce compounds of this nature 

 in the organism. 



On boiling Gelatin in water for a long time we obtain a hydrate of 

 gelatin, which no longer gelatinises : its composition is 4 (C 13 H 10 N 2 



This peculiarity should be remembered, for the compound is likely 

 to be formed in the preparation of broth, and in the application of 

 Papin's Digester to cooking ; and it is regarded by Mulder as doubtful 

 whether this hydrated gelatin can be again converted in the organism 

 into nutrient matter, and whether it may not produce noxious sub- 

 stances in the body. [FOOD.] 



A Gelatin has never yet been discovered in the vegetable kingdom. 

 there is every reason to believe that it is solely produced in the animal 

 body. It is most probably formed from the decomposition of the 

 protein in the blood, through the action of the alkali in the serum, 

 and the oxidising influence of the atmosphere. 



We are likewise imperfectly acquainted with the products of the 

 decomposition of the gelatigenous tissues in the body. Out of the 

 body we know that by the influence of oxidation on gelatin prussic 

 acid is formed, and that, by the action of alkalies, gelatin-sugar, leucin, 

 and extractive matters are produced, while ammonia is disengaged, 

 and an alkaline carbonate formed. Finally, when boiled in dilute 

 sulphuric acid, it yields extractive matters with either gelatin-sugar or 

 leucin. Since leucin is also produced from albumen when decomposed 

 by potash, we perceive an intimate connexion between that protein- 

 compound and gelatinous matters. 



Besides the gelatin obtained from cellular tissue and serous mem- 

 branes there is another kind which has many of its properties, but 

 differs from it in composition. It was first described by Miiller under 

 the name of Chondrin. It is obtained from the cornea, and from those 

 cartilages which do not ossify by boiling them in water. Its compo- 

 ition in 10 (C.,, H M N 4 0,,) + S. 



Gelatin is extensively employed in the arts in the form of glue, and 

 constitutes the basis of leather. It is found pure in the air-bladder 

 of some fishes, and on being cut up constitutes isinglass. When 

 purified it can be formed into plates of almost glass-like transparency, 

 and when coloured is employed for making artificial flowers and many 

 other ornamental objects. Its relation to the other substances in the 

 animal body are considered under PROTEIN. 



GELDER ROSE, or rather, GUELDRES ROSE, a double variety 

 of the Viburnum Opulut, a marsh shrub, common in this country and 

 all the north of Europe. The name of this variety is supposed to 

 indicate its origin in the Low Countries : it is also called the 

 Snowball-Tree, in allusion to its large white balls of flowers. 

 [VlBL-RNUM.I 



GELIDIUM. [ALOJ5.] 



( y-: M M A ST RE A. [M ADREPHYLLKEA.] 



GEMMULINA. [FoRAttiNiFERA.] 



GEMS-BOC. [ANTILOPEA] 



GENERATIONS, ALTERNATION OF. During the course of 

 the development of many of the lower animals from the ovum to 

 their adult condition, they not only pass through various forms, as is 

 een in the Insect tribes [INSECTS], but at certain stages of their growth 

 they posneas the power of multiplying themselves. The individuals 

 which exhibit this phenomenon have been called ' nurses,' and the 

 whole series of phenomena connected with this mode of reproduction 

 have been called by its first expounder, Professor Steeustrup, an 

 ' alternation of generations.' This phenomenon has been particularly 

 observed in tne Acaleplue, Entozoa, Polypi/era, Salpie, and VorticMa. 

 In the various articles on these families of animals, their mode of 

 development is described. As however this subject is one of general 

 interest, and very imperfectly understood, we take the opportunity o 



reproducing here Professor Steenstrup's general remarks on this 

 subject, from a translation of his work published by the Ray 

 Society : 



" The mode of development by means of ( nurses,' or intermediate 

 generations, is thus seen to be no longer an isolated phenomenon in 

 nature. The circumstance of an animal giving birtk to a progeny 

 permanently dissimilar to its parent, but which itself produces a new 

 generation, which either itself or in its offspring returns to the form 

 of the parent animal, is a phenomenon not confined to a single class 

 or series of animals ; the vertebrate class is the only one in which it 

 has not yet been observed. It would consequently appear that there 

 is something intrinsic in this mode of development, and that it occurs 

 as it were with a certain necessity ; on which account it will undoubt- 

 edly soon be recognised to a greater extent and more generally. It 

 should no longer be considered as something paradoxical or anoma- 

 lous (as we have hitherto been too much inclined to deem both it and 

 the phenomena in which it is exhibited), it must be iu harmony with 

 the rest of development in nature, in which the fundamental principle 

 of this course of development must also be elsewhere expressed, 

 although it may be displayed in a form under which we shall less 

 readily perceive and recognise it. This is seen when we trace the 

 mode of development in question more widely through nature ; and 

 whilst contemplating- it through the phenomena in which it is mani- 

 fested, we comprehend it in -its true light. 



" If we collect and regard in one view the whole system of develop- 

 ment by means of 'nursing' generations, as it is exhibited in the 

 Bell-Shaped Polypes (Campanularia), the Claviform Polypes (Coryne), 

 Medusce, Salpa, Vorticellce, and Entozoa, it appears as a peculiar and 

 consequently as an essential feature in this course of development, 

 that the species (that is, the species iu its development) is not wholly 

 represented in the solitary, full-grown, fertile individual- 1 * of both 

 sexes, nor in their development ; but that to complete this repre- 

 sentation, supplementary individuals, as it were, of one or of several 

 precedent generations are requisite. Thus, the distinction between 

 this course of development and that which is generally recognised in 

 nature, in which the species is represented by the individual (of both 

 sexes) and its development, is the want on the part of the individuals 

 of a complete individuality as representatives of the species, or of a 

 specific individuality, if I may so express it. If now we agree to 

 regard such an incompleteness in the individual as the essence of this 

 development, we shall comprehend its significance in nature when we 

 thoroughly consider this course of development in its various periods, 

 throughout the above-mentioned families, how it begins and advances, 

 so that at last we discover to what it tends. I believe, also, that we 

 might trace even now this development by means of precedent, pre- 

 paratory generations of ' nurses ' in its peculiar course and advance, 

 notwithstanding the paucity of instances adduced in the foregoing 

 pages, and the many gaps in the series of observations. Thus we see 

 the greatest incompleteness and the highest degree of mutual depend- 

 ence in the Campamtlarice and similar Polypes, in which the genera- 

 tions representing the unity of the species are very unlike each other, 

 and in which all the individuals are fused, as it were, into an outward 

 unity, or into a set of Polypes. They exist, organically connected 

 with each other, and are normally free only in their firat generation, 

 and indeed only in their earliest stage of development, and only for a 

 short time, since the free-swimming ciliated embryo swims about iu 

 the water at most for some hours, in order to find a suitable place for 

 the foundation of a new polype stem. In the Corynce, or claviform 

 Polypes, the organic connection between the individuals and genera- 

 tions is rather more lax; the perfect gemmiparous or ovigerous indi- 

 viduals are usually quite free, often even at an early age (Coryne 

 fritillariO', Corymorpha), so that they do not attain their full develop- 

 ment until after their separation from the ' nursing ' generation. In 

 the Medvjce and Salpre, the generations which are connected together 

 into one whole, become more like each other ; the firat generation of 

 the Medusae is still fixed but more active and mobile iu its parts ; the 

 individuals of the perfect generation leave the 'nursing' animal while 

 still very small, and undergo remarkable changes after they have 

 become free and are swimming freely about ; both generations of the 

 Salpce, finally, are free, and free awimmers, only the individuals of one 

 of them are organically connected with each other ; they have how- 

 ever no common organs (in the full-grown state), and if my explanation 

 of the alternate generation of the compound Ascidians is correct, we 

 have in that instance precisely the development of the Salpce at a 

 somewhat lower stage ; the individuals of the one generation are 

 organically connected, without having a common organ ; but both 

 generations are fixed. 



" In the class of Entozoa a similar progressive attempt at becoming 

 free and accomplishing a perfect growth appears evident to me. 



" In the Cestoidea the generation of perfect individuals, constitutes 

 externally a unity ; they are only successively detached from each 

 other as the term of their existence approaches, and their whole 

 existence is throughout connected with the ' nursing ' animal. In 

 some of the Trematoda, the later generations remain witliin the earlier 

 until they have attained their full development ; in others they forsake 

 them in an earlier condition, are free, and free swimming, and undergo 

 a complete metamorphosis ; in aorne of these latter, the earlier gene- 

 rations are transformed into motionless, and, as it were, lifeless cysts, 



