254: THE MICROSCOPE AND ITS REVELATIONS. 



cal cell of the Plant ( 223); viz., a definite 'cell-wall,' inclosing cell- 

 contents (of which the nature may be very diverse), and also including a 

 ' nucleus/ which is the seat of its formative activity. It is of such cells, 

 retaining more or less of their characteristic spheroidal shape, that every 

 mass of fat, whether large or small, is chiefly made up (Fig. 468). And 

 the internal cavities of the body are lined by a layer of epithelium- cells 

 (Fig. 466), which, although of flattened form, present the like combina- 

 tion of components. But there is a large number of cases in which the 

 cell shows itself in a form of much less complete development; the ' ele- 

 mentary part ' being a corpuscle of protoplasm, of which the exterior has 

 undergone a slight consolidation, like that which constitutes the ' pri- 

 mordial utricle ' of the Vegetable cell ( 223) or the 'ectosarc' of the 

 Amoeba ( 403), but in which there is no proper distinction between 

 ' cell- wall ' and ' cell-contents.' This condition, which is characteristi- 

 cally exhibited by the nearly globular colorless corpuscles of the Blood 

 ( 666), appears to be common to all cells in the incipient stage of their 

 formation: and the progress of their development consists in the gradual 

 differentiation of their parts, the ' cell-wall' becoming distinctly sepa- 

 rated from the * cell-contents,' and these from the 'nucleus; ' and the ori- 

 ginal protoplasm being very commonly replaced more or less completely 

 by some special product (such as fat in the cells of adipose tissue, or 

 haemoglobin in the red corpuscles of the blood), in which cases the 

 nucleus often disappears altogether. In the earlier stages of cell-develop- 

 ment, multiplication takes place with great activity by a duplicative sub- 

 division that corresponds in all essential particulars with that of the Plant- 

 cell ( 226); as is well seen in Cartilage, a section of which will often 

 exhibit in one view the successive stages of the process 1 (compare Fig. 

 470 with Fig. 139). Whether 'free' cell-multiplication ever takes place 

 in the higher Animals, is at present uncertain. 



651. A large part of the fabric of the higher Animals, however, is 

 made up of fibrous tissues, which serve to bind together the other com- 

 ponents, and which, when consolidated by calcareous deposit, constitute 

 the substance of the skeleton. In these, the relation of the ' germinal 

 matter ' and the ' formed material ' presents itself under an aspect which 

 seems at first sight very different from that just described. A careful 

 examination, however, of those 'connective-tissue-corpuscles' (Fig. 461) 

 that have long been distinguished in the midst of the fibres of which 

 these tissues are made up, shows that they are the equivalents of the 

 corpuscles of 'germinal matter,' which in the previous instance came to 

 constitute cell-nuclei; and that the fibres hold the same relation to them, 

 that the 'walls' and ' contents' of cells do to their germinal corpuscles. 

 The transition from the one type to the other is well seen in Fibro-cartilage, 

 in which the so-called ' intercellular substance ' is often as fibrous as ten- 

 don. The difference between the two types, in fact, seems essentially 

 to consist in this that, whilst the segments of ' germinal matter ' which 

 form the cell-nuclei in cartilage (Fig. 470) and in other cellular tissues, 

 are completely isolated from each other, each being completely sur- 



1 Great attention has lately been given by many able observers, to the changes 

 which take place in the nucleus before and during its cleavage. A full account 

 of these is contained in the recently-published third Edition of Prof. Strassbur- 

 ger's " Zellbildung und Zelltheilung " (1880). See also Dr. Klein's ' Observations 

 on the Structure of Cells and Nuclei,' in "Quart. Journ. Microsc. Science," N.S., 

 Vol. xviii. (1878), p. 315, and Vol. xix. (1879), pp. 125, 404; and Chap, xliv. of his 

 " Atlas of Histology." 



