94 Drs. Kelaarfc and Mobius on the Natural History of 



described fine dark lines of the nacreous layers take part in their 

 formation, and especially in giving them their finely punctured 

 appearance when cut transversely. 



In one section from a sea-pearl of good lustre, the nacreous 

 layers are penetrated almost all round by dark ramifications 

 arranged radially. These have also been found by Carpenter in 

 many shells ; as they do not disappear with the carbonate of 

 lime, he regards them as walled (ausgekleidete) canals. 



The finest pearls usually consist only of nacreous layers sur- 

 rounding a nucleus. To this their white colour is due, as a 

 large dark nucleus shines through the layers, and renders them 

 dull. The grey tint of the so-called immature pearls is due to 

 columnar cells, identical with those of the middle layer of the 

 shells, which extend radially from the nucleus to the nacreous 

 layers. In transverse section these appear as three- or six-sided 

 cells with straight or curved outlines, and they either retain the 

 same diameter throughout, or have their extremities pointed and 

 wedged in between each other. When the lime is removed by 

 acetic acid, they remain as hollow columns, with brownish mem- 

 branous walls. In the midst of them there often rises a 

 membranous plate, which looks like a cell-nucleus, and which 

 is attached to a delicate membrane situated between the columnar 

 and nacreous layers. The membranous walls of the columnar 

 cells are either transversely striated, or furnished with transverse 

 rows of pores ; and in one case the pores and striae occur toge- 

 ther in the same column. Carpenter regards these columns as 

 consisting of flat cells laid one upon the other, and we may sup- 

 pose that the fusion of these takes place in the same way as in 

 the formation of the vessels of plants. This view has certainly 

 more in its favour than that of Heinrich Meckel*, who supposes 

 that the columnar layer is produced from the nacre in the same 

 way as Reaumur's porcelain from amorphous glass. 



In many pearls the inner extremity of the columns rests upon 

 a brownish -yellow mass which encloses the nucleus. It is usually 

 homogeneous, rarely stratified, and possesses all the properties 

 of the epidermis of the shells. This leads us to the considera- 

 tion of the formation of pearls and its causes, upon which so 

 many theories have been propounded. 



According to the ancient Indian notion adopted by Pliny, 

 and after him by many others, the mollusk is incited to the pro- 

 duction of pearls by the falling of drops of dew into its gaping 

 valves ; and so firmly had this idea taken possession of the 

 human mind, that Columbus, on discovering the coast of Paria, 

 thought he had fallen upon the right place for pearls, — the 

 trees grew with their roots in the sea, and these very roots were 

 * Mikrogeologie, p. 26, 1856. 



