FORMATION OV THE SKELETON IN THE MADKEPOKARIA. 477 



of calcification, around which the calicoblasts are grouped in 

 the living polyp, and from which, therefore, similarly oriented 

 fibres ultimately radiate when complete calcification has taken 

 place" (1. c, p. 128; refer to p. 133, fig. 18; p. 148, fig. 29, 

 etc.). 



The wider spacing or closer massing of the unit elements 

 in the skeletal layer was thus shown to depend upon localisa- 

 tion of "centres of calcification" in the ectoderm, and the 

 number produced in a single growth-period to be greatest 

 where fissional activity was most marked in the ectodermal 

 layer. Careful measurements conducted over many species 

 farther showed that the size of the " calicoblast cells" in the 

 ectoderm was the same as the size of the unit elements in 

 the skeletal layer (p. 117, 136, etc.). Upon such grounds "I 

 took the nuclear fission to be associated with the separation 

 of the organic outer layer" containing the skeletal elements, 

 and concluded that, in virtue of fissional processes in the 

 cellular tissue of the ectoderm, calcifying calicoblasts were 

 constantly being eliminated, and the vitality of the ectoderm 

 itself renewed. 



"The fibre-containing calicoblasts which lie next the 

 skeleton are shed off, so to speak, from the polyp, new cells 

 constantly taking their place in the ectoderm by cell-divi- 

 sion" (Ogilvie, 1. c, p. 102). 



" At the beginning of any particular groAvth-period the 

 calcification goes on only locally at certain points of the 

 calicoblastic layer of the colony " (id., 1. c, p. 131 ; cf. p. 143). 



" The calicoblasts remain adherent to one another in dense 

 groups or may be more uniformly distributed. And in this 

 manner they are gradually left behind on the skeleton, and 

 completely calcify, while active cell-division develops con- 

 stantly new ectodermal cells. The calicoblasts adherent to 

 the skeleton represent such as were already in course of 

 losing living continuity with the polyp at the time when the 

 polyp Avas removed from the skeleton" (id., 1. c, p. 116). 



" The scale-like arrangement on the surface presents irre- 

 gularities — sometimes like thicker zones, sometimes thicker 



