2. SYMMETRICAL CELL DEVELOPMENT IN 

 THE FAVOSITID^E* 



(PLATES XXXII AND XXXIII) 



THE majority of compound corals included in the Favositidse 

 are composed of polygonal prismatic cells or corallites in juxta- 

 position. When, however, these cells become free, their form 

 is cylindrical. The polygonal form of closely arranged cells 

 is therefore explained as the natural result of crowding. 



The species Pleurodictyum lenticular e Hall, sp., is an 

 example of simple cell growth and multiplication. In the 

 development of this species,, as shown by the writer in 

 the previous paper, the initial corallite is first conical. The 

 growth of a peripheral series of buds results in changing the 

 sub-circular section of the parent corallite into a polygon. 

 The buds are angular on the sides in juxtaposition to the 

 parent cell and adjacent buds, but on the free portion of 

 their periphery they are cylindrical. The subsequent growth 

 of peripheral buds brings the first series wholly within the 

 corallum, and they are then polygonal in section like the 

 parent corallite. 



In compact corals with long cell tubes, as Michelinia and 

 Favosites, there is a maximum limit to the size of the coral- 

 lites. Thus the form of the cells which have reached this 

 limit of diametral extension is that of equal hexagonal prisms. 

 This is of course due to the well-known fact of six equal 

 tangent circles about a central circle of the same size. Then 

 from crowding, or from the elimination of the interstitial 

 spaces, they assume a regular hexagonal form. The speci- 



* Trans. Conn. Acad. Sci., VIII, 215-220, pis. xiv, xv, 1891. 



