. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 490 TEGUMENTARY ORGANS. certain that any given spot of the mantle of a young bivalve must give origin, directly or indirectly, first, to "cpiderm;" 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
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 490 TEGUMENTARY ORGANS. certain that any given spot of the mantle of a young bivalve must give origin, directly or indirectly, first, to "cpiderm;" 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. 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 (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 {infrequently 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 intercell
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