322 DR. H. LYSTER JAMESON ON 
depositions or ealcospherules * arise in the tissues, close to the 
attachments of the muscles to the shell. Ectoderm-cells may 
“ migrate to the source of irritation, and thus be responsible for 
the deposition of a pearl.” No explanation of the origin of these 
calcospherules is given, but Mr. Southwell thinks it is ‘almost 
certain that they are depositions from the blood,” and refers to 
them elsewhere as “ of excretory origin ” (42). 
I have been led by my observations to take a quite different 
view of these ‘ calcospherules” +, and as their origin is so closely 
related to that of Muscle-Pearls, | cannot do better than begin 
the present section of my paper with an account of their struc- 
ture and origin. 
According to my view, Prof. Herdman’s ‘ calcospherules ” are 
not free concretions at all, but are minute pearls, composed of 
hypostracum ; and I propose, therefore, to call them “ hypostra- 
cum muscle-pearls,” to separate them from “ nacreous muscle- 
pearls.” As stated by Herdman, these bodies occur close under 
the epidermis (unless secondarily displaced, e. g. by the addition 
of new ones), and I usually find them in the region where the 
muscle-attachment epithelium passes over into the ordinary shell- 
secreting epidermis of the mantle. A group of these hypo- 
stracum-pearls is shown on Pl, XX XIX. fig. 22, which represents 
a portion of the mantle-musculature of one of the unlabelled 
specimens in the British Museum, examined entire in oil of 
cloves. The same pearls, decalcified, are seen in fig. 23. These 
little bodies measured from 0:02 to 0°5 mm. in diameter. In 
Pl. XX XVIII. fig. 19 similar bodies, hy.p., are seen in a section 
along with ordinary nacreous muscle-pearls; while single indi- 
viduals are shown in Pl. XX XIX. figs. 21 & 21a@and Pl. XL. 
fig. 25. Sections ground from these bodies, or cut from the organic 
residues left when they are decalcified, show them to be composed 
of the same substance as the hypostracum of the shell. They 
consist of calcium carbonate, in fine fibrocrystalline form, showing 
radial and also concentric markings, with a small central cavity 
(Pl. XX XIX. fig. 21a). Decalcified they also resemble hypo- 
stracum in all details of structure and reaction to stains 
(fig. 21). Their organic basis stains more blue with hematoxylin 
than the organic parts of the other shell-substances, and takes up 
carmine more deeply. Their alveolar structure is also much finer 
than that usually found in the columnar varieties of repair- 
substance, so fine, in fact, that in surface-sections the reticular 
structure seems almost like that of the protoplasm itself. As has 
been observed in the hypostracum of the shell, this substance 
sometimes passes over into nacreous conchyolin laterally. The 
* This word is presumably intended to convey the same idea as the word 
“concretion” adopted by me (25) in 1902, i. e. a spherocrystal-like body arising in 
the tissues otherwise than by epidermal secretion; and therefore analogous to 
cholesterin calculi, etc. (ef. Harting’s “ Calcospherites,” 12). 
+] find that Rubbel (34 a), working on the freshwater Pearl-Mussel, 
Margaritana, has arrived independently at the same view of the nature of these 
bodies as that here propounded. 
[64] 
