324 DR. H- LYSTER JAMESON ON 



80 fi in diameter, and has a wall about 10 /x thick, lies close to a 

 nacreous muscle-pearl, about 1 mm. in diameter, the sac of which 

 is shown at ep.p.s. The cyst is embedded in a sti-and of muscle 

 traversing the mantle-parenchyma obliquely, and ending in muscle- 

 attachment epidermis which was attached to the shell. (Such 

 connections between the general musculature of the mantle and 

 the shell occur here and there quite apart from the more regular 

 muscle-scars. For examples of this in Mytilus see List, 37 b, 

 PI. 8. fig. 1.) The cyst contains at one point a little granular 

 mass. The muscle-fibres here appear to be in direct contact with 

 the hypostracum. The easiest explanation of this condition would 

 seem to be the hypothesis that the original epithelium has dis- 

 appeared. It is not difficult to suppose that a highly specialised 

 " tendinous " epithelium, like the attachment-epidermis, whose 

 fate seems to be to become a part of the shell, is incapable of 

 reo'enerating itself, and, therefore, destined to die and disappear 

 on ceasing to be functional. If we take this view, the typical 

 hypostracum -pearl is not so much a stage in the development of a 

 nacreous pearl as a phase parallel with it ; the latter arising when 

 the original sac contains some of the ordinary nacre-secreting 

 epidermis, or cells capable of giving rise thereto, the former when 

 it is composed of attachment-epithelium alone. The hypostracum- 

 pearl Avould thus have a limited growth, the nacr-eous pearl an 

 unlimited growth. However, in considering these cases where 

 there does "not appear to be any attachment-epithelium, it must 

 be remembered that this particular epithelium is often very difficult 

 to see, so that some workers have even failed to detect its existence 

 on the regular muscle-insertions. Much light can no doubt be 

 thrown on these questions by a really thorough study of the 

 behaviour of the cells at the places where the muscle-attachment 

 epithelium goes over into the ordinary epidermis of the mantle, 

 and of the histological phenomena associated with the wandernig 

 of the muscle-attachment. The material of the pearl-oyster that 

 I have examined so far is not sufficiently well preserved to allow 

 of such study. So far as I know, this important matter has never 

 been properly investigated in any mollusc. 



I will now pass from the hypostracum muscle-pearls to the 

 nacreous muscle-pearls. Typical instances of these are shown in 

 PI. XXXY. figs. 8 & 9 and PI. XXXYI. fig. 10. These three 

 examples are all explicable as derivatives of the hypostracum- 

 pearl. Figs. 8 & 10 obviously lie in the borderland between 

 one of the regular muscles and the parenchyma (fig. 8 is at the 

 insertion of one of the pedal levators). Fig. 9, from one of 

 Prof. Herdman's slides, is in a place in the free mantle where a 

 few small muscle-strands (inusc.) are attached to the shell. The 

 sac of each of these pearls is lined in part by ordinary nacre- 

 secreting epithelium, underlying which is the typical granular 

 parenchyma, in part by muscle-attachment epithelium, continuous 

 with the musculature. As the former is much more active than 

 the latter, these pearls are all eccentric in shape, having a hilum 



