R. KTRKrATRICK. 



bifurcation.^ Accordingly heaps of amceboc^'tes would accu- 

 mulate at these points, and would extend along- lines 

 at right angles to the choanosoraal branches till they 

 met neighbouring ridges, in such a way as to enclose poly- 

 gonal or circular areas, over the edges of which the choano- 

 somal branches would extend (and between the tubercles or 

 points of greatest heaping). At last the shallow meshes 

 would become pits into which the heavy amcebocytes loaded 

 with calcigenons granules would migrate. The heavy masses 

 of loaded ainoebocytes in the pits would tend to stay there 

 but the choanosome and ectosome would continue their 

 growth in the direction of the surface. A tendency to a 

 cleavage into two zones, hypersome and hyposome, would 

 gradually become accentuated, especially in cases where the 

 sponge can increase considerably in depth. 



The result of the pull between opposing forces would be 

 to cause in the little cylinders of soft tissue an hour-glass 

 constriction. The continual growth of cells and tissues 

 in a confined space would iiU in the space round the hour- 

 glass neck so that the '^glasses" would become two closely 

 approximated cylinders, between which the amcebocytes, 

 or rather, calcocytes, would form a tabula leaving only 

 an isthmus of tissue. Often even this little isthmus is nipped 

 in and finnlly cut through by the total closure of the t;i.bul;i, and 

 the calcocytes below become cut off and burled in their crypt. 



Obviously with varying factors there will be varying 

 results. With uniform migration of calcocytes over a flat 

 surface tliere is a tendency to peripheral extension rather 

 than to gi-owth in depth, and a uniform pull between hyper- 

 some and liyposome will lead to uniformity in size of crypts 

 and to deposition of tabulae in horizontal planes, and so on, 



' The laying down of calcareous bars in the earliest stages before the 

 flagellated chambers are formed would seem at first to furnish an argu- 

 ment against the theory that the form of the calcareous skeleton has 

 Vjeen, so to speak, moulded by the choanosome. It must be remembered 

 that siliceous spicules are found, too, in the youngest stage, and these 

 must originally have been developed later than the flagellated chambers. 

 By "choanosome" is here meant chiefly the canal system and flagellated 

 chambers. 



