VIIl] 



THE SEGMENTATION OF A DISC 



371 



walls ; and again, that so long as our partition films are fluid or 

 semifluid, their points and lines of contact with one another may 

 shift, hke the shifting outhnes of a system of soap-bubbles. This 

 is the physical cause of the movements frequently seen among 

 segmenting cells, like those to which Kauber called attention in 

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

 movements or accommodations which give rise to a so-called 

 ''spiral" type of segmentation. 



Bearing in mind, then, these considerations, let us see what 

 our flattened disc is likely to look Hke, after a few successive 



Fig. 153. Diagram of flattened or discoid cell dividing into octants : to shew 

 gradual tendency towards a position of equilibrium. 



divisions into component cells. In Fig. 153, a, we have a diagram- 

 matic representation of our disc, after it has divided into four 

 quadrants, and each of these in turn into a triangular and a 

 quadrilateral portion; but as yet, this figure scarcely suggests 

 to us anything hke the normal look of an aggregate of living cells. 

 But let us go a httle further, still limiting ourselves, however, 

 to the consideration of the eight-celled stage. Wherever one of 

 our radiating partitions meets the peripheral wall, there will (as 

 we know) be a mutual tension between the three convergent films, 

 which will tend to set their edges at equal angles to one another, 

 angles that is to say of 120°. In consequence of this, the outer 

 wall of each individual cell will (in this surface view of our disc) 



24—2 



