330 DR. H. LYSTER JAMESON ON 
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. 
(8) They are typically formed around central cavities in which 
granules may be, but are not necessarily, present, and which may 
be lined in the first instance with hypostracum, ordinary nacre, 
or repair-substances analogous to those which occur where the 
normal shell-secreting processes are disturbed. 
(4) They are often associated with great numbers of little 
bodies, which Herdman calls “ caleospherules,” 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 1s more than 
likely that a great mz any of the pearls which I refer to it ar e, in 
fact, of the same origin as muscle-pearls, but have been produced 
singly at spots w here two or three musele-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 ete.) of the pearl. In consequence of the 
presence of this abnormal shell-substance the centre of a paren- 
chyma-pearl frequently contains a dark, spherical, concentrically 
laminated, radially striated pseudo-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 6), pl. 8. fig. 1. 
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