1-^30 DR. H. LYSTER JAMESON OX 



we call pearl-sacs) in the tissues ; and that consequently the 

 " cause " of the pearl is not to be looked for in the nucleus, which 

 in the Ceylon pearl-oyster frequently does not exist, but rather in 

 the tissues of the oyster. This is, after all, just what I said 

 at the bottom of p. 142 in my 1902 paper. 



The characters of muscle-pearls may be summed up as 

 follows : — 



(1) They usually and probably invariably arise in close associa- 

 tion with the epidermis at the point where muscle-attachment 

 epithelium passes over into normal nacre-secreting epithelium. 



(2) They frequently occur several together or clustered in 

 numbers, 



(3) They are typically formed arovmd central cavities in which 

 granules may be, but are not necessarily, present, and which may 

 be lined in the first instance with hypostracum, oi'dinary nacre, 

 or repair-substances analogous to those which occur where the 

 noi'mal shell-secreting processes are disturbed. 



(4) They are often associated with great numbers of little 

 bodies, which Herdman calls " calcospherules," and which I regard 

 as minute pearls composed of hypostracum. 



(5) They are, according to Herdman, characteristic of certain 

 of the Ceylon beds ; and are, therefore, local in their occurrence, 

 which would give support to a parasitic theory of their origin. 



B. Parenchyma-Pearls (" Cyst-Pearls" Herdman). 



This class contains a much more heterogeneous group of nuclei 

 and pseudo-nuclei than the last. The pearls which it comprises 

 may have arisen from more than one cause, and it is more than 

 likely that a great many of the pearls which I refer to it ai'e, in 

 fact, of the same origin as muscle-pearls, but have been produced 

 singly at spots where two or three muscle-fibres are attached to 

 the shell, instead of in clusters at the regular muscle-insertions, 

 and have consequently assumed a spherical form *. 



Parenchyma-pearls often show a distinct central nucleus con- 

 sisting of granules or masses of dark substance which might be 

 either of parasitic origin or derived from dead tissue-cells (e. g. 

 leucocytes). In some cases the nuclei of these pearls contain 

 or consist of grains of sand or other foreign particles. The 

 nucleus is typically succeeded by one or more layers of repair- 

 substance, which often intergrades with the normal shell- 

 substances (nacre etc.) of the peai-1. In consequence of the 

 presence of this abnormal shell -substance the centre of a paren- 

 chyma-pearl frequently contains a dark, spheiical, concentrically 

 laminated, radially striated pseu.do-nucleus. This opaque pseudo- 

 nucleus has no doubt on many occasions been mistaken for the 

 remains of a parasite, in pearls decalcified and examined entire ; 

 but sections generally reveal its real nature unmistakably. 



*■ These occasional attachments of small muscles to the shell on the general 

 mantle-surface are well known. See e.g. List (27 h), pi. 8. fig. 1. 



