VIIl] 



OF THE SEGMENTATION OF A DISC 



581 



outer border; in oth^r words, our partition may (B) cut both radial 

 walls, or (C) may cut one radial wall and the periphery. These are 

 the two methods of division which Sachs called, respectively, 

 (B) periclinal, and (C) anticlinal*. We may either treat the wa&s 

 of the dividing quadrant as already sohdified, or at least as having 

 a tension compared with which that of the incipient partition film 

 is inconsiderable; in either case the new partition must meet the 

 old wall, on either side, at right angles, and (its own tension and 

 curvature being everywhere uniform) must take the form of a 

 circular arc. 



jgX 



M 



Fig. 232. 



We find that a flattened cell which is approximately a quadrant 

 of a circle invariably divides ^fter the manner of Fig. 231, C, that 

 is to say, by an approximately circular, anticlinal wall, and this 

 we now recognise in the eight-celled stage of Eryihrotrichia (Fig. 

 230) ; let us then consider that Nature has solved our problem, and 

 let us work out the actual geometrio conditions. 



Let the quadrant OAB (in Fig. 232) be divided into two parts 

 of equal area, by the circular arc MP. It is required to determine 



* There is, I think, some ambiguity or disagreement among botanists as to the 

 use of this latter term : the sense in which I am using it, viz. for any partition 

 which meets the outer or peripheral wall at right angles (the strictly radial partition 

 being for the present excluded), is, however, clear. 



