VIII] THE SEGMENTATION OF A DISC 591 



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 parti- 

 tions will arise transversely to their long axes, that is to say, radially 

 (as U, V) ; and we shall consequently have a superficial or peripheral 

 layer of quadrilateral cells, with sides approximately 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. 



In following out these theoretic principles, and others Hke to 

 them, in the actual division of living cells, we must 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 Unks,. and very important links, 

 in a chain of physical causation ; they are always £tt work, but their 

 effects may be overridden and concealed by the operation of other 

 forces. Secondly, we must remember that, in most cases, the cell- 

 system which we have in view is constantly increasing in magnitude 

 by active growth; and by this means the form and also the propor- 

 tions of the cells are continually altering, 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 walls; and again, that so long as our partition films are 

 fluid or semifluid, their points and fines of contact with one another 

 may shift, like the shifting outlines of a system of soap-bubbles. 

 This is the physical cause of the movements frequently seen among 

 segmenting cells, like those to which Rauber called attention in 

 the segmenting ovum of the frog, and hke those more striking 

 movements or accommodations which give rise to a so-called 

 "spiral" type of segmentation. 



Bearing in mind these considerations, let us see what our fla|itened 

 disc is hkely to look like, after a few successive Cell-divisions. In 

 Fig. 239 a, we have a diagrammatic representation of our disc, after 

 it has divided into four quadrants, and each quadrant into a 

 triangular and a quadrilateral portion; but as yet, this, figure has 



