626 

 radii* 



THE FORMS OF TISSUES 



[CH. 



In Fig. 274 I have divided a circle into its four quadrants, 

 and have bisected each quadrant by a circular arc {BC), passing from 

 radius to periphery, as in the foregoing cases of cell-division (e.g. 

 p. 590); and I have again bisected, in a similar way, the triangular 



B^ \B 



Fig. 274. Diagrammatic arrangement of partitions, represented by skeletal 

 rods, in a larval Echinoderm (Ophiura). 



halves of each quadrant (D, D). I have also inserted a small circle in 

 the middle of the figure, concentric with the large one. If now we 

 imagine the partition-hnes in the figure to be replaced by solid 

 rods, we shall have at once the frame-work of an Ophiurid (Pluteus) 



Fig. 275. Pluteus-larva of Ophiurid. 



larva. Let us imagine all these arms to be bent symmetrically 

 downwards, so that the plane of the paper is transformed into 

 a conical surface with curved sides; let a membrane be spread, 

 umbrella-Hke, between the outstretched skeletal rods, and let 



* J. Loeb has shewn (Amer. Journ. Physiol, vii, p. 441, 1900) that the sea-urchin's 

 egg can be reared for a time in a balanced solution of sodium, potassium and 

 calcium chloride, developing no spicules and so forming no pluteus larva; on 

 adding sodium carbonate the spicules are laid down and the pluteus larva takes 

 shape accordingly. 



