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a little water, covered with a thin slip of glass, and examined by 

 transmitted light with a power of 280 linear, it presents the appear- 

 ance of a thin, yellow-coloured, horny membrane, thickest at its junc- 

 tion with the old lip of the shell, and becoming gradually thinner 

 towards the outer margin. There are an infinite number of minute 

 globular vesicles scattered over the whole of its surface, which vary 

 in diameter from 7-5-0-^ to ?&* of an inch, (PI. xiv. fig. 1). These 

 vesicular bodies are incipient cytoblasts and cells ; for, although in 

 the early stage of their growth we cannot discern the nucleus, they 

 may be readily traced through their various stages of development, 

 until in the larger vesicles it may frequently be observed ; and the 

 young cells approaching each other are compressed closely together, 

 and become the first superficial layer of cellular structure. It fre- 

 quently happens that in the process of mounting a piece of the young 

 lip of the shell for examination, a number of the cytoblasts are dis- 

 lodged from the new membrane. If these be examined with a power 

 of 600 linear, the nucleus is very distinctly to be seen in the greater 

 part of them, and the new cells may be observed in various stages of 

 development, with their accompanying cytoblasts, as represented in 

 PI. xiv. fig. 3. The completion of the process of the development 

 of the cellular tissue may be best observed in the immediate neigh- 

 bourhood of the old lip of the shell, and thence gradually towards 

 the outer margin of the newly-formed membrane, they may be seen 

 in all their different stages. While the young cells are detached from 

 each other, and retain their globular forms, they are perfectly trans- 

 parent, the secretion of calcareous or other matters not appearing to 

 have commenced ; but we frequently observe, in the neighbourhood 

 of the newly-formed stratum, small patches of cells congregated to- 

 gether, which are of a deep yellow tinge, and of a semi-opaque ap- 

 pearance, as if they were separate centres of ossification, arising in 

 the same manner that we observe in the production of the calcareous 

 matter in the young cartilages of the bone in the higher classes of 

 the animal kingdom. Amid the great mass of vesicular bodies des- 

 tined to form the cells for the secretion of the calcareous deposits of 

 the shell, we find other cytoblasts dispersed over all parts of the 

 membrane, which are developed in the form of tessellated cellular 

 structure, by which means the original membrane is very much in- 

 creased in substance. This production of tessellated cellular structure 

 is more particularly abundant where the calcigerous cells are in their 

 advanced stages of development. If a portion of the newly-formed 

 stratum of shell be removed from the membrane upon which it reposes,. 



