-6 THE PERIODIC GROWTH OF SCALES IN GADIDvE 



As to the mode of formation of these canaliculi, Eaudelot remarks that he has 

 not a sufficiently large number of facts to give a satisfactory answer to this 

 question. He says, however, that where radiating canals exist, the canaliculi 

 form themselves on their course, at their free extremities on the posterior 

 border of the scale. At the extremity of the radiating groove there firstly 

 appears a small depression ; later by the mode of growth of the surrounding 

 tissue this depression deepens more and more, and finally closes in at the 

 posterior end, forming an aperture like a minute pierced gap across the lamina 

 of the scale, which is very thin at this point. As the scale increases by the 

 addition of new layers to its internal face, each gap become gradually con- 

 verted into a narrow canal, in which the length varies with the thickness of 

 the scale and with the distance of the canaliculi from the posterior border. 



As to the nature and function of the perforating canaliculi, Baudelot throws 

 out certain hints. He believes that the canaliculi give passage to a fila- 

 mentous cord, Avhich is either of the nature of connective tissue or a nerve- 

 fibre. He is inclined to believe that it is of the latter nature, and if this is 

 true, that there might be grounds for establishing a connection between the 

 perforating canaliculi and the canals which traverse the scales of the lateral 

 line. The scales of the lateral line receive nerve-fibres on their deep surface, 

 and in their interior nervous structures have been demonstrated. 



In Mugil, in all those scales showing similar passages to those of the lateral 

 line, a certain number of perforating canaliculi anastomose with the median 

 canal of each scale. 



In the pike, many of the scales have a similar trench to those of the lateral- 

 line scales. This trench, hollowed out at first, may be considered as analo- 

 gous to the depressions which represent the first stage in the formation of 

 the perforating canaliculi. In a sparoid fish showing a disjointed and equilant 

 lateral line he found a scale which showed at its centre of growth a duct which 

 penetrated obliquely from the internal to the external face of the scale. This 

 duct, while much narrower than the median canal of the lateral line, was at the 

 same time very mucli larger than the perforating canaliculi of adjoining parts, 

 that is to say, a kind of transition between the two kinds of canals. From the 

 preceding facts, which he throws out in passing, Eaudelot thinks that if they 

 are confirmed by later researches, a clear resemblance between scales of the 

 lateral line and other scales would become apparent. This would also explain 

 why in certain types all scales or a large number of them may revert to the 

 characteristic features of scales from the lateral line. 



8. The internal laacna; of scales. Certain scales possess lacunar developed 

 in their interior. In Holocentrum longipenne, for example, some of the perfo- 

 rating canaliculi show lateral diverticula which spread out horizontally in the 

 scale substance. These diverticula constitute a system of lacunae. In the 

 scales of Hypostoma internal lacunar are well developed, and constitute a 

 vast system of anastomosing canals, in which the cavity communicates with 

 those of the spines. In Daclijloi>teriis volitans the scales arc hollowed out in 

 their central portions by large irregular lacunae which communicate with each 

 other. In the tunny the scales present remarkable lacunae. In this case 

 these lacunae, which occupy all the median portion of the scale, form a species 



