TEGUMENTARY ORGANS 



399' 



{Mm 



m 



we find the cells neither in contact with each other, nor polygonal in 

 form, but separated by a greater or less amount of intercellular 

 substance, and presenting a rounded, instead of an angular form {fig. 

 314. C). Upon looking still nearer the margin, the cells are seen tO' 

 be yet smaller and more separated by intercellular substance, and not 

 unfrequently we lose all trace of distinct cells, the intercellular sub- 

 stance presenting itself alone, but con- 

 taining cytoblasts scattered through 

 it. This appearance has been noticed 

 by myself in Pinna and Unio, and by 

 Mr. Bowerbank in Ostrea ; so that I 

 have no doubt that it is general in 

 this situation. We may, I think, con- 

 clude from it that the cells of the 

 prismatic cellular substance are devel- 

 oped, like those of cartilage, in the 

 midst of an intercellular substance, 

 which at first separates them from 

 each other, that as they grow and 

 draw into themselves the carbonate 

 of lime poured out from the sub- 

 jacent surface, they approach each 

 other more and more nearly ; and 

 that, as they attain their full develop- 

 ment, their sides press against each 

 other, so that the cells acquire a poly- 

 gonal form, and the intercellular 

 substance disappears." 



I have given Dr. Carpenter's 

 statement at length, because it ap- 

 pears to me to express very distinctly 

 the interpretation which one is at 

 once tempted to put upon the appear- 

 ance, but which I must reject for the 

 following reasons : — In the first place, if we examine that portion: 

 (a) of the margin beyond the smallest granules {cytoblasts, Car- 

 penter), it is seen to be either absolutely structureless or obscurely 

 striated, not a trace of a cell or endoplast being anywhere visible. 

 Secondly, if any dilute acid be added under the microscope, the 

 apparent nuclei and cells vanish with effervescence, and leave behind 

 them clear empty spaces, of exactly the same shape and size as they 

 themselves had. Thirdly, the supposed cells have a peculiar concen- 



FlG. 313. — A to c, Unio ; D, Helix. 



