406 



THE POPULAR EDUCATOR. 



or germ cell, and a sperm or fertilising cell, are always neces- 

 sary. Sometimes both products are developed in the same 

 individual, which is then said to be monoecious or hermaphrodite, 

 as the Ccelenterata, certain Scolecidse, trenmtode Entozoa, 

 Medusae, etc. In snails and other pulmogasteropods, there is 

 an interchange of office. In the remaining animals, which are 

 termed dioecious, the reproductive elements are found in separate 

 individuals belonging to opposite sexes. In fishes and am- 

 phibia the ova are fertilised without the body, as already 

 described ; in reptiles, birds, and mammals, within the body of 

 the ovigerous or female parent. From this fertilised ovum the 

 embryo is developed. For a brief review of this process we 

 cannot do better than quote the following words of Carpenter : 

 " The earliest part in the history of embryonic development is 

 nearly the same in all animals ; for it consists in the multipli- 

 cation of the simple cell of which the original germ is com- 

 posed, untfl a cluster is formed, all the cells of which appear to 

 be in every respect similar to one another. Each of these cells 

 either takes into itself, or draws around it, a portion of the 

 vitellus or yolk, which is the nutrient substance of the ovum ; 

 and thus either the whole or a portion of this vitellus is sub- 

 divided into a number of minute spherules, altogether consti- 

 tuting what is known as the mulberry mass. Among the 

 Invertebrata generally, the embryo comes forth from the egg in 

 a very simple condition, a large part of its structure having 

 undergone but little change from the state of the ' mulberry 

 mass,' and in these the whole yolk undergoes subdivision. 

 The same is the case, too, in the batrachian reptiles, which 

 issue from the egg in a form very different from that into which 

 they are to be subsequently developed. And it is the case even 

 with Mammalia, but for a very different reason, their embryonic 

 structure, first formed at the expense of the yolk, being destined 

 to acquire additional material for its full development from a 

 source altogether different. In the highest mollusks, however, 

 as also in fishes, ordinary reptiles, and birds, the portion of the 

 yolk which undergoes subdivision is comparatively small ; and 

 the great mass of the vitellus is destined to be subsequently 

 absorbed into the substance of the germ. When the whole of 

 the yolk is taken into the mulberry mass, the formation of the 

 embryo is usually the result of the progressive metamorphosis 

 of its parts, the cells of its surface being converted into integu- 

 ment, and those of its inner part into the internal organs. 

 This is the case, for example, in the intestinal worm. The 

 embryonic condition of many of the organs is frequently re- 

 tained at the time when the young animal comes forth from the 

 egg, those parts only being completed which are necessary to 

 enable it to obtain its nutriment. Other organs are subse- 

 quently evolved at the expense of the food thus introduced ; 

 and thus a complete change or metamorphosis may take place, 

 in regard alike to external form and to internal structure, 

 between the larval and the adult states. Of this phenomenon 

 wo have characteristic examples in the groups of Insects and 

 Batrachia. The change is sometimes gradual, as in the pro- 

 gressive advance of the tadpole into the condition of the frog. 

 But it is sometimes apparently sudden, as when the chrysalis 

 skin is thrown off, and the perfect insect comes forth. In the 

 latter case, however, the change is just as gradual as in the 

 former, since the organs characteristic of the perfect insect are 

 undergoing development during the whole of the chrysalis 

 period." 



In animals supplied with a food-yolk the embryo is nourished 

 until it has nearly acquired its adult condition, although far 

 from having attained its adult size (Carpenter). 



In the lowest forms of the Invertebrata the ova are developed 

 in loose filamentous tissue, or in membranous folds, or upon 

 stalks, or processes in the interior of the body, as in the Ccelen- 

 terata, or are actually embedded in its substance, as in the 

 Protozoa. In the higher forms the ovaries consist of sacs, 

 caeca, or tubuli, which may be simple or ramified, and which 

 have an attached duct, named the oviduct. In fishes the ovaries 

 are double and symmetrical, as previously described. They are 

 double in the Amphibia, and the ova are brought to maturity 

 simultaneously, not in succession, as in reptiles, birds, and 

 mammals ; being received into the oviducts, they are conveyed 

 to the cloaca, and then deposited into the water, either singly 

 or in masses, and there undergo development. In reptiles, as 

 in birds, the ovary is usually simple. In the latter the Dorking 

 fowl is an exception to this rule. In the embryonic condition 



of birds both ovaries are present ; the right one, however, dis- 

 appears. The ovary consists of a number of small spherical 

 bodies named ova (Fig. VI., 1, page 265), invested by delicate 

 membranes called ovisacs (2). To the lower part of the ovary 

 is attached the infundibulum (5), or wide funnel-shaped opening 

 of a single tortuous tube called the oviduct (6, 6) ; this termi- 

 nates below in the cloaca (12), which, as before mentioned, 

 constitutes the common outlet of the alimentary, urinary, and 

 reproductive organs. 



For the successful incubation of birds' eggs a steady and 

 continuous temperature of about 100 Fahr. is requisite. The 

 period of incubation varies, in different birds, from ten to fifty 

 or sixty days. In mammals the ovaries are double, one on each 

 side of the uterus, to which they are connected by means of a 

 duct called the Fallopian tube, which corresponds to the oviduct 

 of birds, and through which the ovum passes into the uterus, 

 where it remains until it is sufficiently developed. 



We have described the most important features in the animal 

 kingdom, step by step, from those organisms which constitute 

 the simplest forms of life, to those more complex beings which 

 constitute the highest. The student will have observed already 

 that the different members are marvellously connected. In 

 fact, we might exclaim with Faust, in the sublime language of 

 Goethe 



" How all things In a whole here weave and blend, 

 One in the other working, moving, living." 



Or with Huxley, that " Every animal has a something in com- 

 mon with its fellow ; much with many of them ; more with a 

 few ; and usually so much with several that it differs but little 

 from them." As the same talented writer has further asserted,* 

 " The powers or faculties of all kinds of living matter, diverse 

 as they may be in degree, are substantially similar in kind." 

 Again, " A threefold unity pervades the whole living world 

 namely, a unity of power, or faculty, a unity of form, and a 

 unity of substantial composition." 



The bodies of animals are seemingly made up of a variety of 

 tissues, which differ from each other ; and, so far as the unaided 

 eye can judge, such is the case. But microscopic analysis 

 clearly and indisputably demonstrates the wonderful fact, that 

 the relationship which these tissues bear to each other, in 

 their early condition, is not so far removed as we should 

 otherwise imagine. All are alike made up of a number of 

 cells, constituting protoplasm, or what has been termed the 

 physical basis of life. It is by the combination and super- 

 addition of these cells that tissues are formed and animals 

 built up. How or why that power which we term life 

 originates, and by what laws it so admirably and persistently 

 arranges its materials, so as to constitute a living being, is 

 beyond man's power of understanding to conceive. It is this 

 life manifested to our senses under such an immense variety of 

 conditions which, while it interests, yet astounds us. Hum- 

 boldt truly remarks : " When the active spirit of man is 

 directed to the investigation of nature, or when, in imagination, 

 he scans the vast fields of organic creation, among the varied 

 emotions excited in his mind there is none more profound or 

 vivid than that awakened by the universal profusion of life." 

 The whole globe is a swarming and teeming world of living 

 things, amid which man himself, also a living being, conducts 

 those operations which justly earn for him the reputation of the 

 most intellectual and intelligent of all animals. It matters not 

 whether we turn to the extensive regions comprised under the 

 name of Siberia, characterised by its three degrees of cold and 

 its Arctic tracts doomed to everlasting snow, or to tho tropical 

 verdure and fertility of the plains of the Indus and the Ganges, 

 and the large sandy or gravelly deserts situated between these 

 plains, doomed to utter barrenness, or to lands favoured with 

 the blessings of a temperate clime in each and every situation 

 we find life existing, adapted by structure and general confi- 

 guration for that sphere in which it may be found. But if 

 land is thus tenanted, what shall we say of the vast expanse 

 of waters which enter so largely into the composition of the 

 globe ? The Arctic and Antarctic, the Indian and Atlantic, 

 and the mighty Pacific Oceans, have each their countless mil- 

 lions of living beings. Even the microscopic medusa? exist in 

 such vast numbers that Scoresby intimates that it would require 



F oriniglitty Review. 



