4-90 



TEGUMENTARY ORGANS. 



certain that any given spot of the mantle of 

 a young bivalve must give origin, directly or 

 indirectly, first, to "epiderra;" secondly, to 

 prismatic substance ; and, thirdly, to nacreous 

 substance ; so that, on examining the free 

 edge of a growing shell, we ought, since the 

 " epiderm " is structureless and transparent, 

 to be able to observe the gradual formation of 

 the prismatic substance upon its under sur- 

 face. This is, in fact, the case. Fig. 313, A, 

 represents such a free edge of the shell of 

 Anodon, a being the direction of the flexible 

 zone ; b, that of the perfect prismatic sub- 

 stance. 



Fig. 313. 



A to c, Unio ; D, Helix. 



Dr. Carpenter describes the appearances 

 here figured in the following terms (/. c. p. 8.) : 



" Although the prismatic cellular structure 

 has not yet been actually observed in process 

 of formation, yet certain appearances, which 

 are occasionally met with in the marginal 

 portions of its newest layers, throw great 

 light upon its mode of growth, and indicate 

 its strong resemblance to cartilage in this 

 respect ; for in these situations 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 (^^.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 containing 

 cytoblasts scattered through it. This appear- 

 ance 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, conclude from it 

 that the cells of the prismatic cellular sub- 

 stance are developed, 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 

 subjacent surface, they approach each other 

 more and more nearly ; and that, as they 

 attain their full development, their sides press 

 against each other, so that the cells acquire a 

 polygonal form, and the intercellular substance 

 disappears." 



I have given Dr. Carpenter's statement at 

 length, because it appears to me to express 

 very distinctly the interpretation which one is 

 at once tempted to put upon the appearance, 

 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, Carpenter), 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 micro- 

 scope, 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- 

 trically or radially-striated structure, resem- 

 bling sections of urinary calculi on a small 

 scale, and still more the corresponding bodies 

 in the integument of the shrimp (suprd.} 

 For these reasons I think it must be granted 

 that the appearances in question, how- 

 ever cell-like, are, in reality, not the expres- 

 sion of the development of a cellular struc- 

 ture at all, but merely that of the mode in 

 which the deposit of calcareous matter takes 

 place in the membranous basis of the shell. 

 In fact, I believe that the calcareous matter 

 appears first in small and distinct globules 

 (the " cytoblasts"), and that more or less 

 concentric deposits take place round these, 

 the result of which is, that the membranous 

 basis is more and more displaced, and that the 

 deposited masses eventually come almost into 

 contact. The regularity of the ultimate pris- 

 matic structure results from that of the dis- 

 tances of the granules primarily deposited, 

 and the even rate of addition to each subse- 

 quently. 



There appears to me to be but one inter- 

 pretation to be placed upon these facts ; viz. 

 that cells as such do not enter into the for- 

 mation of the shell of the Naiades at all, but 

 that it is constituted by the successive excre- 

 tion of membranous laminae from the surface 



