296 BULLETIN OF THE BUREAU OF FISHERIES. 



disagree with this, holding that since the scale is covered with a membrane constantly 

 lubricated with mucus the wear on the hard scale would not be enough to make a 

 noticeable difference. They account for it by a difference of growth. 



Baudelot (1873) admits his inability to give an adequate explanation of the focus. 

 He describes it with minute detail in all its variations and concludes that it and the 

 annuli are due to the same cause but he can not tell what that cause is. 



Hoffbauer (1898, 1900) calls it the center of growth representing the oldest part of 

 the scale and also notes that in some cases it is abnormally large, but that at other parts 

 of the same fish it is normal — observations in perfect accord with Dahl's explanation. 



According to Tims (1906) " it consists of a flattened plate of calcified tissue, 

 elliptical in shape with an irregular margin. From its appearance in section and from 

 a surface view I believe it to be formed of a fusion of a number of basal plates, the 

 spines of which have entirely disappeared." 



Dahl (191 1 ) gives an explanation of abnormally large foci which seems to be ade- 

 quate. Those scales \vith such foci are simply regenerated scales, the focus being 

 composed of secreted matter which filled the empty scale pocket following the loss of 

 the old scale. He illustrates this with a cut, which shows his evidence to be quite 

 conclusive. 



PERFORATING CANALICULI AND INTERNAL LACUNA. 



The observation by Blanchard (1866) of certain very small perforations through 

 which water might freely flow seems to be the origin of the theory that scales have a 

 respiratory function. He noticed that they are especially prominent in the Cyprinidae. 



They were first described by Baudelot, who was able to make out their structure 

 clearly. He describes them as extremely small canals perforating the scale from top 

 to bottom. They are found on the posterior side and pass through the scale obliquely — 

 i. e., from the exterior surface they incline toward the periphery on the posterior side 

 and from the longitudinal axis of the scale. They are developed from notches that 

 appear in the posterior margin and, as the scale grows in size, the notches are sur- 

 rounded by newly secreted substance becoming canals which extend through the entire 

 thickness of the scale. As to function Baudelot thinks that ner\'es pass through them, 

 thus relating them to the supposed sensory function of the lateral line. 



In the interior of the scale the canaliculi may become more or less expanded, form- 

 ing a cavity which, in extreme cases, pervades almost the entire scale between the 

 inferior and superior layers. He called these lacunae. In the case of Dactylopterus 

 voliians the lacunce are very large and are filled with a bony tissue which he regards as 

 a connecting link between scales and bones. He confesses that he is unable to conjec- 

 ture any function for them. 



STRUCTURE, FORMATION, AND GROWTH." 



Agassiz (1834) believed scales to be analogous to nails and hair and hence not 

 living tissue. He explained growth as taking place by secretions from the floor of the 

 scale pocket and by the increasing size of the scale pocket, which enlarges in proportion 

 to the size of the body of the fish. The laminae were different leaves or folia. As to 

 calcification, he regarded it as nonhomogeneous — i. e., occurring in corpuscles or cal- 

 careous bodies — which he believed to occur only on the superior and inferior surfaces. 



<^ For a review of the older literature ou scales, see Thomson, 1904. 



