390 THE FORMS OF TISSUES [ch. 



In the two examples figured (Fig. 174), both comparatively 

 simple ones, it will be seen that, of the main chambers, one is in 

 each case an unsymmetrical one ; that is to say, there is one 

 chamber which is in contact with a greater number of its neighbours 

 than any other, and which at an earlier stage must have had 

 contact with them all ; this was the case of our type /, in the 

 eight-celled system (Fig. 158). Such an asymmetrical chamber 

 (which may occur in a system of any number of cells greater than 

 six), constitutes what is known to students of the Coelenterata as 

 a "fossula"; and we may recognise it not only here, but also in 

 Zaphrentis and its allies, and in a good many other corals besides. 

 Moreover certain corals are described as having more than one 

 fossula : this appearance being naturally produced under certain 

 of the other asymmetrical variations of normal space-partitioning. 

 Where a single fossula occurs, we are usually told that it is a 

 symptom of " bilaterality " ; and this is in turn interpreted as 

 an indication of a higher grade of organisation than is implied 

 in the purely "radial symmetry" of the commoner types of coral. 

 The mathematical aspect of the case gives no warrant for this 

 interpretation. 



Let us carefully notice (lest we run the risk of confusing two 

 distinct problems) that the space-partitioning of Heterophyllia 

 by no means agrees with the details of that which we have studied 

 in (for instance) the case of the developing disc of Erythrotrichia : 

 the difference simply being that Heterophylha illustrates the 

 general case of cell-partitioning as Plateau and Van Rees studied 

 it, while in Erythrotrichia, and in our other embryological and 

 histological instances, we have found ourselves justified in making 

 the additional assumption that each new partition divided a cell 

 into co-equal parts. No such law holds in Heterophylha, whose 

 case is essentially different from the others : inasmuch as the 

 chambers whose partition we are discussing in the coral are mere 

 empty spaces (empty save for the mere access of sea-water) ; while 

 in our histological and embryological instances, we were speaking 

 of the division of a cellular unit of living protoplasm. Accordingly, 

 among other differences, the "transverse" or "periclinal" parti- 

 tions, which w^ere bound to appear at regular intervals and in 

 definite positions, when co-equal bisection was a feature of the 



