CELLS IN RELATION TO TISSUE-FORMATION. xxi 



common occurrence exhibit changes of figure and movements of an amoebine 

 character which seem naturally to be referred to the present head. 



The fact above mentioned, that these movements of cells may be excited 

 by stimulation of the nerves, is especially worthy of note, in as much as it 

 proves that operations effected in and by cells are more or less under the 

 governance of the nervous system. Moreover, the well-known influence of 

 mental states over the secretions, and the effects resulting from ex- 

 perimental stimulation of the nerves of secreting glands, although doubtless 

 due in part to changes in the blood-vessels, seem to show that this subjec- 

 tion to the nervous system extends even to the chemical and physical 

 operations which take place in secreting cells. A curious and interesting 

 observation in proof of this is adduced by Kolliker. He found that the 

 light of the firefly, fampyriSf is emitted from cells in which albuminoid 

 matter is decomposed with production of urate of ammonia, and that the 

 emission of light could be brought on or rendered more vivid by electrical 

 and other stimuli operating through the nerves. 



The well-known tremulous movement which so often affects minute 

 particles of matter, is not unfrequently observed in the molecular contents 

 of cells ; but this phenomenon depends simply upon physical conditions, and 

 is of a totally different character from the motions of the protoplasm above 

 referred to. 



Of Cells in Relation to the Formation of Textures. In a certain sense it 

 might be said that the foundation of all the textures and organs of the 

 body is formed of cells ; for in the early embryo all the different parts of 

 the body, so far as they are characterised by form and position, are made 

 up of embryonic cells ; but how the different and special characters of the 

 fully organised textures are produced is still a matter of no little uncer- 

 tainty. No doubt certain textures in their mature condition are composed 

 of cells, which, on the supposition of continuous cell-generation, must be 

 the progeny of the embryonic cells, although modified in character to suit 

 them to their special destination. The epidermis, the several varieties of 

 epithelium, and the plain or non-striated muscular tissue are examples of 

 this. Other textures or textural elements are not made up of cells or 

 formed immediately from cells, as, for example, the fibres of connective 

 tissue, which are produced in intercellular substance. But there remain 

 cases of tissue-formation in which, whilst it is generally admitted that cells 

 or nuclei take part, it is not agreed to what extent or in what precise 

 mode these bodies share in the process : the development of the fibres 

 of voluntary muscles and nerves affords an illustration of this, as will be 

 afterwards shown in its proper place. Accordingly, the reasonableness of 

 certain limitations and qualifications of the doctrine of tissue-development 

 from cells has been recognised by judicious histologists, even from its first 

 promulgation. 



Of the changes which cells or their elements undergo in the formation of 

 tissues, or afterwards in the exercise of their functions when constituting 

 part of the living organism, the following are what may be considered as 

 best ascertained ; and it is to be observed that the same cell may undergo 

 more than one of the changes indicated. 



1. Cells may increase in size and change their figure. When a cell 

 grows equally or nearly so in all directions, it preserves its globular shape ; 

 but more commonly the growth is greater in one dimension than in another, 

 and then the cell becomes flattened, in which case it may remain as a 

 round disk or change into an oval, fusiform, or strap-shaped figure. When 



