PEARL PRODUCTION. 27 



In i dusion, then, we still adhere to the view we expressed in 1903, that in cyst 



pearls containing an organic nucleus the pearl-secreting epithelium is of ectodermal 

 origin. 



Muscle Pearls. 



There are some pearls, however, that show no nucleus whatever, either organic or 

 inorganic, and it seems probable that these have been formed by the deposition of 

 calcareous matter around a minute calculus in the tissues. These are the pearls that 

 we have distinguished ('Brit. Assoc. Report, Southport,' p. 695) as "Muscle pearls," 

 since we find them most abundantly in the muscular tissue near the insertions of the 

 levator and pallia! muscles. Figs. 5A and 5B, illustrate the distribution of cyst pearls 



Fig. 5A. Diagram showing the comparative 

 frequency of position of cyst pearls in 

 the various parts of the mantle. 



Fig. 5iS. Diagram showing the positions 

 most frequently occupied by muscle 

 pearls. 



and muscle jiearls respectively, the localities most commonly occupied by the pearls 

 being indicated by sj3ots. Figs. 13 and 14 on Plate III. show also the mode of 

 occurrence of the muscle pearls near the insertion scars. 



The muscle pearls when present are usually abundant,"" and when examining under 

 the microscope a young pearl of this kind, in situ, it is common to find a large number of 

 minute calcareous depositions, or calcospherules (Plate III., tig. 12), scattered in the 

 neighbouring tissue. It is probable that the muscle pearls are formed around these 

 microscopic calcospherules as centres of irritation, and as these positions are invariably 

 in our experience close to the surface of the muscle or the mantle, there is no difficulty 

 in understanding that there, if anywhere, ectoderm cells might migrate to the source 

 of irritation and thus be responsible for the deposition of a pearl. 



* At the insertion of one levator muscle, 23 small pearls were counted with the eye, while under the 

 microscope 170 additional tiny spherules were found to be present. 



E 2 



