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NATURE 



[September 6, 1900 



cells are concerned, in the formation of their characteristic pro- 

 ducts. As regards cell reproduction also, though the process of 

 division begins in the nucleus in its chromosome constituents, 

 the achromatic figure in the cell plasm undoubtedly plays a 

 part, and the cell plasm itself ultimately undergoes cleavage. 



A few years ago the tendency amongst biologists was to 

 ignore or attach but little importance to the physiological use of 

 the nucleus in the nucleated cell, and to regard the protoplasm 

 as the essential and active constituent of living matter ; so much 

 so, indeed, was this the case that independent organisms re- 

 garded as distinct species were described as consisting of proto- 

 plasm destitute of a nucleus ; also that scraps of protoplasm 

 separated from larger nucleated masses could, when isolated, 

 exhibit vital phenomena. There is reason to believe that a 

 fragment of protoplasm, when isolated from the nucleus of a 

 cell, though retaining its contractility and capable of nourishing 

 itself for a short time, cannot increase in amount, act as a 

 secreting structure, or reproduce its kind : it soon loses its 

 activity, withers, and dies. In order that these qualities of 

 living matter should be retained, a nucleus is by most ob- 

 servers regarded as necessary (Nussbaum, Gruber, Haberlandt, 

 Korschelt), and for the complete manifestation of vital activity 

 both nucleus and cell plasm are required. 



Bacteria. 



The observations of Cohn, made about thirty years ago, and 

 those of De Bary shortly afterwards, brought into notice a group 

 of organisms to which the name " bacterium " or " microbe " is 

 given. They were seen to vary in shape : some were rounded 

 specks called cocci, others were straight rods called bacilli, 

 others were curved or spiral rods, vibrios or spirill?e. All were 

 characterised by their extreme minuteness, and required for 

 their examination the highest powers of the best microscopes. 

 Many bacteria measure in their least diameter not more than 

 i/25000th of an inch, i/ioth the diameter of a human white blood 

 corpuscle. Through the researches of Pasteur, Lord Lister, 

 Koch, and other observers, bacteria have been shown to play 

 an important part in nature. They exercise a very remarkable 

 power over organic substances, especially those which are com- 

 plex in chemical constitution, and can resolve them into simpler 

 combinations. Owing to this property, some bacteria are of 

 great economic value, and without their agency many of our 

 industries could not be pursued ; others again, and these are the 

 most talked of, exercise a malign influence in the production of 

 the most deadly diseases which afflict man and the domestic 

 animals. , 



Great attention has been given to the structure of bacteria and 

 to their mode of propagation. When examined in the living 

 state and magnified about 2000 times, a bacterium appears as a 

 homogeneous particle, with a sharp definite outline, though a 

 membranous envelope or wall, distinct from the body of the 

 bacterium, cannot at first be recognised ; but when treated with 

 reagents a membranous envelope appears, the presence of which, 

 without doubt, gives precision of form to the bacterium. The 

 substance within the membrane contains granules which can be 

 dyed with colouring agents. Owing to their extreme minute- 

 ness it is difficult to pronounce an opinion on the nature of the 

 chromatine granules and the substance in which they lie. Some 

 observers regard them as nuclear material, invested by only a 

 thin layer of protoplasm, on which view a bacterium would be a 

 nucleated cell. Others consider the bacterium as formed of 

 protoplasm containing granules capable of being coloured, which 

 are a part of the protoplasm itself, and not a nuclear substance. 

 On the latter view, bacteria would consist of cell plasm inclosed 

 in a membrane and destitute of a nucleus. Whatever be the 

 nature of the granule-containing material, each bacterium is 

 regarded as a cell, the minutest and simplest living particle 

 capable of an independent existence that has not yet been 

 discovered. 



Bacteria cells, like cells generally, can reproduce their kind. 

 They multiply by simple fission, probably with an ingrowth of 

 the cell wall, but without the karyokinetic phenomena observed 

 in nucleated cells. Each cell gives rise to two daughter cells, 

 which may for a time remain attached to each other and form a 

 cluster or a chain, or they may separate and become independent 

 isolated cells. The multiplication, under favourable conditions 

 of light, air, temperature, moisture and food, goes on with ex- 

 traordinary rapidity,- so that in a few hours many thousand rtew 

 individuals may arise from a parent bacterium. 



Connected with the life-history of a bacterium cell is the 



KO I 6 10. VOL. 62] 



formation in its substance, in many species and under certain 

 conditions, of a highly refractile shiny particle called a spore. 

 At first sight a spore seems as if it were the nucleus of the 

 bacterium cell, but it is not always present when multiplication 

 by cleavage is taking place, and when present it does not appear 

 to take part in the fission. On the other hand, a spore, from 

 the character of its envelope, possesses great power of resistance, 

 so that dried bacteria, when placed in conditions favourable to 

 germination, can through their spores germinate and resume an 

 active existence. Spore formation seems, therefore, to be a 

 provision for continuing the life of the bacterium under con- 

 ditions which, if spores had not formed, would have been the 

 cause of its death. 



The time has gone by to search for the origin of living 

 organisms by a spontaneous aggregation of molecules in vegetable 

 or other infusions, or from a layer of formless primordial slime 

 diffused over the bed of the ocean. Living matter during our 

 epoch has been, and continues to be, derived from pre-existing 

 living matter, even when it possesses the simplicity of structure 

 of a bacterium, and the morphological unit is the cell. 



Development of the Egg. 



As the future of the entire organism lies in the fertilised egg 

 cell, we may now briefly review the arrangements, consequent 

 on the process of segmentation, which lead to the formation, 

 let us say in the egg of a bird, of the embryo of the young 

 chick. 



In the latter part of the last century, C. F. Wolff observed that 

 the beginning of the embryo was associated with the formation 

 of layers, and in 1817 Pander demonstrated that in the hen's egg 

 at first one layer, called mucous, appeared, then a second or 

 serous layer, to be followed by a third, intermediate or vascular 

 layer. In 1828 von Baer amplified our knowledge in his 

 famous treatise, which from its grasp of the subject created a 

 new epoch in the science of embryology. It was not, however, 

 until the discovery by Schwann of cells as constant factors in 

 the structure of animals and in their relation to development 

 that the true nature of these layers was determined. We now 

 know that each layer consists of cells, and that all the tissues 

 and organs of the body are derived from them. Numerous 

 observers have devoted themselves for many years to the study 

 of each layer, with the view of determining the part which it 

 takes in the formation of the constituent parts of the body, more 

 especially in the higher animals, and the important conclusion 

 has been arrived at that each kind of tissue invariably arises 

 from one of these layers and from no other. 



The layer of cells which contributes, both as regards the 

 number and variety of the tissues derived from it, most largely 

 to the formation of the body is the middle layer or mesoblast. 

 From it the skeleton, the muscles, and other locomotor organs, 

 the true skin, the vascular system, including the blood and other 

 structures which I need not detail, take their rise. From the 

 inner layer of cells or hypoblast, the principal derivatives are 

 the epithelial lining of the alimentary canal and of the glands 

 which open into it, and the epithelial lining of the air-passages. 

 The outer or epiblast layer of cells gives origin to the epidermis 

 or scarf skin and to the nervous system. It is interesting to 

 note that from the same layer of the embryo arise parts so 

 different in importance as the cuticle — a mere protecting struc- 

 ture, which is constantly being shed when the skin is subjected 

 to the friction of a towel or the clothes— and the nervous system, 

 including the brain, the most highly differentiated system in the 

 animal body. How completely the cells from which they are 

 derived had diverged from each other in the course of their 

 differentiation in structure and properties is shown by the fact 

 that the cells of the epidermis are continually engaged in repro- 

 ducing new cells to replace those which are shed, whilst the 

 cells of the nervous system have apparently lost the power of 

 reproducing their kind. 



In the early stage of the development of the egg, the cells in 

 a given layer resemble each other in form, and, as far as can be 

 judged from their appearance, are alike in structure and pro- 

 perties. As the development proceeds, the cells begin to show 

 differences in character, and in the course of time the tissues 

 which arise in each layer differentiate from each other and can 

 be readily recognised by the observer. To use the language of 

 von Baer, a generalised structure has become specialised, and 

 each of the special tissues produced exhibits its own structure 

 and properties. These changes^re coincident with a rapid 



