514 



THE FORMS OF TISSUES 



[Un. 



When, as happens in certain corals, the peripheral walls or 

 "thecae" of the individual polypes remain undeveloped but the 

 radiating septa are formed and calcified, then we obtain new and 

 beautiful mathematical configurations (Fig. 194). For the radiating 

 septa are no longer confined to the circular or hexagonal bounds of 

 a polypite, but tend to meet and become confluent with their 

 neighbours on every side; and, tending to assume positions of 

 equihbrium, or of minimum, under the restraints to which they 

 are subject, they fall into congruent curves, which correspond in 

 a striking manner to lines running in a common field of force 

 between a number of secondary centres. Similar patterns may be 

 produced in various ways by the play of osmotic or magnetic forces ; 



Fig. 194. Surface-views of corals with undeveloped thecae and confluent septa. 

 A, Thamnastraea ; B, Comoseris. From Nicholson, after Zittel. 



and a very curious case is to be found in those compHcated forms 

 of nuclear division known as triasters, polyasters, etc., whose 

 relation to a field of force Hartog in part explained*. It is obvious 

 that in our corals these curving septa are all orthogonal to the 

 non-existent hexagonal boundaries ; and, as the phenomenon is due 

 to the imperfect development, or non-existence, of a thecal wall, it 

 is not surprising that we find identical configurations among various 

 corals, or families of corals, not otherwise related to one another. 

 We find the same or very similar patterns displayed, for instance, 

 inSynhelia (Oculinidae), in Philhpsastraea (Rugosa), in Thamnastraea 

 (Fungida), and in many more. 



* Cf. M. Hartog, The dual force of the dividing cell, Science Progress (N.S.), 

 I, Oct. 1907, and other papers. Also Baltzer, Mehrpolige Mitosen bet Seeeigeleiern, 

 Diss., 1908. ' 



