NATURE OF ANIMAL CELLULAR TISSUES. 691 



539. The Cells of which many Animal -tissues are essentially 

 composed, consist, when fully and completely formed, of the same 

 parts as the typical Cell of the Plant (§ 182); — viz., a definite 

 'cell-wall,' enclosing '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 (§ 564). And the 

 internal cavities of the body are lined by a layer of Epithelium 

 Cells (§ 563), which, although of flattened form, present the like 

 combination 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 'elementary part ' being a corpuscle of protoplasm 

 or 'germinal matter,' of which the exterior has undergone a 

 slight consolidation, like that which constitutes the 'primordial 

 utricle' of the Vegetable Cell (§ 183) or the 'ectosarc' of the 

 Amceba (§ 331), but in which there is no proper distinction of 

 'cell-wall,' 'cell-contents,' or nucleus. This condition, which is 

 characteristically exhibited by the nearly-globular Colourless Cor- 

 puscles of the Blood (§ 556), appears to be common to all Cells in the 

 incipient stage of their formation ; and the progress of their develop- 

 ment consists in the gradual differentiation of their parts, the ' cell- 

 wall' and 'cell-contents' being separated (as 'formed material') from 

 the 'germinal-matter,' which last usually remains as the 'nucleus,' — 

 generally, however, contracting, and sometimes ( when its work has been 

 completely done) disappearing altogether. The large flattened Red 

 Corpuscles of the Blood of the Frog and other Oviparous Yertebrata 

 (§ 555) appear to be generated from the Colourless by the production 

 of a layer of the 'formed material ' chemically known as Hsemato- 

 globulin around the original protoplasmic particles. For Corpuscles 

 are met with, which seem to constitute an intermediate sta.se 

 between the two kinds; their form being still globular, but their 

 size being greater than that of the Colourless corpuscles; whilst 

 their peripheral portion shows a distinct layer of 'formed material,' 

 which is beginning to assume the characteristic hue of the Red 

 disk, but which is not tinged by the Carmine-solution that deeply 

 dyes the central or nuclear portion. This 'formed material,' 

 however, does not seem ever to acquire a distinct membranous 

 envelope or cell-wall ; the changes of shape which the Red Corpus- 

 cles spontaneously undergo under favourable circumstances, being 

 such as could scarcely occur if their form were thus limited. In 

 Cartilage (§ 565), on the other hand, the 'nucleus' and the 'cell- 

 contents' are completely differentiated from the 'cell-wall ;' but the 

 1 cell-wall ' itself cannot be separated from the ' intercellular sub- 



or specialized Tissues of the higher Animals, and the undifferentiated 

 Sarcode-substance which alone represents those in the lowest, he has 

 explained himself more fully in the Fourth Edition (1865) of his '* Manual 

 of Physiology." 



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