Morphological Part. 



Calcification. 



Dififerent views. Different modes of calcification. 



IN his well-known paper on Membranipora membranacea H. Nitsche' expresses 

 the view that calcification proceeds in the cuticle given off by tlie cells of 

 the covering membrane, which at the places wliere such deposition takes place 

 is divided into three layers and it is the middle layer characterized by its highly 

 refracting power which is impregnated by the calcium salts. Nitsche arrived at 

 this result exclusively from an investigation of the not yet calcified short spines, 

 of which in this species one occurs at each of the posterior corners of the zocecium. 

 According to Nitsche these spines are covered externally by a thick cuticle and 

 beneath this there is a highly refracting layer intended for calcification but which 

 does not quite reach out to the tip of the spine. Internally the whole cavity of 

 the spine is covered by an exceedingly thin membrane, which according to 

 Nitsche is the innermost layer of the cuticle, whilst he calls a net-work of 

 cellular strings in the inner cavity the endocyst. On this view however there 

 would be no continuous covering of endocyst in the interior of the spine, which 

 does not seem very probable and it would therefore be most natural to call the 

 thin, innermost layer of the wall as the endocyst and the cellular net-work as a 

 portion of this endocyst, which has been designated the endosarc or the mesen- 

 chyme. I have not had the opportunity to investigate an uncalcified spine of 

 Membr. membranacea, but if Nitsche's theory of the calcification is correct the 

 spines when complelety calcified must be covered with a cuticle. I have sought 

 for this in vain however, both on the spines mentioned and on all other spines 

 examined by me and I must therefore contest the correctness of Nitsche's view 

 that the lime particles are deposited in a middle layer of the cuticle. 



1 80. pp. 42. 76. 



