556 IV. D. Lang — Beriiard'n Biological Theories of Fomls. 



of a pile of the minute specks of calcium carbonate and reposing on 

 the end of a bundle of liniii threads. We have thus reached a smaller 

 skeletal unit than the granule, namely a micro-granule, secreted at 

 the end of a linin-thread-bundle. A pile of these forms a skeletal 

 fibre ; a bundle of fibres forms a granule which exists as a separate 

 entity in virtue of the chalicoblasts being differentiated as cells, 

 i.e. though connected by linin filaments the cytoplasm is arranged in 

 groups each dominated by its several nucleus. Except, however, at the 

 surface where the skeleton has only just been secreted, the outlines of 

 the granules vanish, and the skeleton has a fibrous structure alone. 



Any section of a fossil coral, if well enough preserved, shows 

 a fibrous sti'ucture (I am not aware that fossils are ever so well 

 preserved that the lines of growth described by Ogilvie in recent 

 corals appear), and the septa invariably, and other elements variously, 

 have a median (generally) dark line. This dark line is the expression 

 of the secreting surface folded upon itself ; each side of the fold 

 secretes the fibrous skeleton, and the line marks the junction of the 

 two layers. The skeletsil parts other than the septa may be interpreted 

 as secreted by a double or single ectodermal surface according to 

 whether they have or have not the median line. Ogilvie explains 

 how complex folding of the secreting surface causes the skeletal fibres 

 to appear rather to radiate from centres than to be pinnately arranged 

 on each side of the median line. 



We have now seen that Bernard's theory explains the fibrous coral 

 skeleton whether the fibres appear in the section of a fossil or as 

 composing a granule isolated (as ac^hieved by Ogilvie) from the septal 

 surface of a recent coral. It is also conceivable that, if secretion were 

 quickly taking phue, calcium carbonate might be deposited just 

 beneath the surface of the cytoplasm round the end of a linin-thread- 

 bundle as well as at its end, and so pieces of cytoplasm might thus 

 get entangled in the general skeletal layer and give the appearance 

 that suggested to Heidt-r and Ogilvie that a whole chalicoblast was 

 convened into a granule of calcium carbonate. 



Periodicity in evolution is the second principle just mentioned as 

 standing out from Bernard's work. It is here suggested that this 

 principle has a far wider application than that given it by Bernard. 

 Demonstrated by him as evident in the evolution of animal phyla, 

 periodicity is here claimed as bound up also with ontogeny and 

 possiblj' related to tachygenesis.' 



Wilson has shown in the development of the coral Ilanicitm'^ that 

 the first six mesenteries appear singly, though the last four of these 

 follow so closely upon one another as rather to resemble the entrance 

 of a pair of pairs. In other Ccelenterates the mesenteries have been 

 described as appearing in pairs or in cycles. The first mesenteric 

 period of Mnnicina, then, may be called the ' single ' period, since its 

 structural unit is a single mesentery ; its first phase (anabasis') is the 

 production of the first two mesenteries, one after the other ; its 



' Hyatt, " Bioplastology and the related branches of Biologic Eesearch " : 

 Proc. JBoston Soc. Nat. Hist., vol. xxvi, p. 77, 1893. 



' Wilson, " On the Development of Manicina areoJata''^ : Journ. Morph., 

 vol. ii, pp. 191-248, 1889. 



