370 



THE FORMS OF TISSUES 



[CH. 



triangle, PRQ, the least width is near the innermost angle, that 

 at Q ; and the bisecting circular arc will therefore be opposite to Q, 

 or (approximately) parallel to PR. The importance of this fact is 

 at once evident; for it means to say that there soon comes a 

 time when, whether by the division of triangles or of quadrilaterals, 

 we find only quadrilateral cells adjoining the periphery of our 

 circular disc. In the subsequent division of these quadrilaterals, 

 the partitions will arise transversely to their long axes, that is to 

 say, radially (as U , V) ; and we shall consequently have a super- 

 ficial or peripheral layer of quadrilateral cells, with sides approxi- 

 mately parallel, that is to say what we are accustomed to call an 

 epidermis. And this epidermis or superficial layer will be* in clear 

 contrast with the more irregularly shaped cells, the products of 

 triangles and quadrilaterals, which make up the deeper, underlying 

 layers of tissue. 



Fig. 152. 



In following out these theoretic principles and others like to 

 them, in the actual division of living cells, we must always bear 

 in mind certain conditions and qualifications. In the first place, 

 the law of minimal area and the other rules which we have arrived 

 at are not absolute but relative : they are links, and very important 

 links, in a chain of physical causation ; they are always at work, 

 but their effects may be overridden and concealed by the operation 

 of other forces. Secondly, we must remember that, in the great 

 majority of cases, the cell-system which we have in view is con- 

 stantly increasing in magnitude by active growth ; and by this 

 means the form and also the proportions of the cells are continually 

 liable to alteration, of which phenomenon we have already had 

 an example. Thirdly, we must carefully remember that, until 

 our cell-walls become absolutely solid and rigid, they are always 

 apt to be modified in form owing to the tension of the adjacent 



