AS AN INDEX OF ACxE. 27 



of spongy tissue, limited at each side, namely, on the external and internal 

 faces, by a thin plate of compact tissue. As to the mode of development of 

 these lacuna?, Baudelot admits the absence of all knoAvledge ; but he thinks 

 that the " presence of these lacuna) in the tissue of certain scales establishes 

 a clear analogy between the structure of these productions and that of osseous 

 tissue." The lacunro of the scales of the tunny and of Dactylopterus, for 

 example, resemble very completely the lacunae seen in the ossified connective 

 tissue of the rays in the fins of various fishes (Gasterosteus, etc.). Kamifying 

 lacunae, such as are found in the scales of Holocentrum, can be observed with 

 exactly similar characters in the operculum and suboperculum of the same 

 fish, and as in the scales, the lacunae of the opercular bones communicate 

 with the exterior by ducts analogous to the perforating canaliculi. " These 

 facts seem to show that the phylogeny of scales and that of osseous tissue 

 should be associated." 



9. The focus or centre of growth. Under this term one understands that 

 more or less central part of the scale around which growth first takes place. 

 In the rigorous meaning of the word the focus ought to be represented by a 

 point which corresponds to the exact spot of origin of the scale ; but in using 

 this term zoologists have given this word a wider meaning, namely, that 

 region of the scale in which formation first takes place in the life of the 

 fish, and which is characterised by the absence of or irregularity of the con- 

 centric ridges. In some scales the focus is smooth or only very slightly 

 roughened ; in others its surface is marked by projecting calcareous reliefs, 

 granulations, or tubercles, either laid down in lines or without any definite 

 order; in others, again, ridges analogous to concentric ridges occur, which by 

 their indefinite arrangement form an inextricable netAvork, or a network of 

 irregular meshes. The focus, as a rule, shows no grooves ; but in some cases 

 the radiating grooves are prolonged to the focus, sometimes retaining their 

 original characters, sometimes, however, becoming interrupted from point to 

 point, and thus forming small superficial lacunae which are not disposed in 

 any regular order. When these grooves reach the centre of growth (focus) 

 they frequently anastomose with those of the opposed border. It is frequently 

 difficult to define the precise limits of the focus, owing to the fact that an 

 insensible transition is effected to the surrounding parts. The dimensions of 

 the focus, however, show very great variations, not only in scales of difi'erent 

 types of fish, but in scales from the same fish. 



In the perch, minnow, and pike, liaudelot has shown how the dimensions of 

 the focus may vary in the scales of the same fish : by the side of scales in which 

 the focus is almost nil, one may meet with others in which the focus attains the 

 size of one-half to two-thirds the total diameter of the scale. This fact alone is 

 sufficient to demonstrate that the size of the focus is not proportioned to that of 

 the scale. Some scales possess a very large focus, those of Labrus and Crenilabrus 

 for example; others, on the contrary, possess a very small focus. The position 

 of the focus in relation to the centre of the scale is very variable from one type 

 to another. In some fish the focus occupies nearly the centre of the scale ; 

 this is the case, for example, in the lote, minnow, and eel ; this fact appears 

 more especially true when scales are small, rounded and concealed in the 



