139 



in which we see embedded, without any apparent order, and at un- 

 certain distances from each other, numerous incipient cytoblasts. As 

 we continue our examination inwards, we find these in all the various 

 stages of growth, until the partially developed cells are seen assuming 

 an approximation towards an arrangement, although not yet in actual 

 contact, and presenting the appearance represented by fig. 3, PI. xvi. 

 At the bases of many of the cells the cytoblasts are to be seen in 

 their fullest state of development. At this period the axes of the 

 cells appear to be at right angles to the plane of the membrane upon 

 which they are seated, but as they are developed in succession layer 

 after layer beneath each other, until the stratum is six, seven, or more 

 cells deep, the outer ones assuming a diagonal direction, their apices 

 pointing outwards and backwards, as represented in PI. xvi. fig. 4. 

 As the cells reach their fully developed state, their membranous 

 structure becomes too thick to allow of a distinct view of the cyto- 

 blasts which are seated at their bases ; but in the young and immature 

 cells these organs are very distinctly to be seen, provided the observer 

 focuses the bases and not the apices of the cells. 



If the fragments of the lamina? intended for examination be torn 

 off in such a manner as to carry with them portions of the inner sur- 

 face of the shell, it frequently occurs that this membrane is seen to 

 be beautifully studded with matured cytoblasts, but without cells 

 having been developed. These organs in Ostrea are different from 

 any which I have observed in other shells, in which we usually ob- 

 serve them to assume either a circular or an oval form, but in this 

 shell their fully developed form appears to be that of a rhomboid, as 

 represented attached to the cells in fig. 3, PI. xvi. The membranous 

 structure which separates each stratum of cells from those above and 

 beneath it, is thick, semi-opaque, and has a fleshy appearance. It 

 abounds in Haversian canals, which average q 6 £ 6 inch in diameter. 

 Sometimes they pursue their courses through this tissue in nearly a 

 straight line, for a considerable distance, without branching or anas- 

 tomosing, while in other parts they are tortuous, frequently anastomose, 

 and throw off numerous short branches, which have coecoid termina- 

 tions, as represented in fig. 5, PI. xvi., with the cellular structure 

 imperfectly visible beneath the yellow semi-opaque membrane. The 

 best mode of obtaining a satisfactory view of these organs is by the 

 maceration of parts of the shell in weak hydrochloric acid ; and if 

 the animal matter thus obtained be carefully separated into laminae, 

 the canals, and the thin, pellucid, membranous structure of the cells, 

 may be readily and successfully examined in a fluid medium, by 



