48 TIIK PEHIODIC GROWTH OF SCALES IN GADID.1^ 



Ussow concludes liis paper in the following words : — 



" The scale of Teleosteans is a plate consisting of two layers. The upper 

 layer (including the relief) consists of homogeneous tissue witliout any struc- 

 ture (frequently one can, however, observe on focussing a slight striation 

 running parallel to the upper surface of the scale). This layer originates 

 at the expense of the mesoderm elements of the dermis, at the expense of 

 the so-called scleroblasts ; its inorganic portion consists of amorphous calcium 

 phosphate. The tissue of this layer is an ordinary simple bony tissue. 



"The lower layer of the scale also originates at the expense of the same 

 mesoderm elements. It is formed in part out of indurated (sclerosirten) 

 connective-tissue fibrillse, between which no cells exist (in the species investi- 

 gated by me). One terms its tissue a tense connective tissue." 



The next paper which I notice, that by Hoff bauer* on the "Age-determination 

 of carp by means of their scale-!," bears more distinctly on my own contribu- 

 tions on the scales of marine fishes than any of the papers previously men- 

 tioned. Dr. Hoffbauer's work is that from which my investigations had 

 their origin, and I would therefore lay all due stress upon it. This 

 author showed that carp bred in pond or aquarium, for all of which he 

 had exact knowledge as to their age and history, possessed in their scales a 

 means of age-determination. 



The scale of a carp shows the following structure : — There are two distinct 

 areas, firstly, a non-transparent part covered by the upper skin, and 

 situated towards the tail of the fish, which may be termed the posterior 

 field or area ; and a transparent part enveloped in the scale pocket, which may 

 be called the anterior field or area. Only the anterior area comes into con- 

 sideration in the determination of age. This anterior field, as distinguished 

 from the posterior, shows fine lines which run approximately parallel to the 

 margin of the scale, and apparently take their origin from a more or less 

 median point lying back towards the posterior area of the scale. This median 

 point is the centre of growth, and constitutes the oldest portion of the scale, 

 characterised by the absence of lines or stride, lie describes these lines or 

 striae in the anterior area of the scale as concentric lines or striae, which 

 show much similarity in their arrangement to the annual rings seen in the 

 transverse sections of trees. The concentric lines consist of ridge-like 

 elevations of the upper surface of the scale, in consequence of which the 

 upper surface is rougher to the touch than the under surface. The formation 

 of these concentric lines has a very close connection with the growth of the 

 scale. According to Baudelot, the lines are due to the fact that the lower 

 surface of the scale, consisting oidy at first of a small thin plate (the centre 

 of growth), gradually lays down lamella? which always become larger in 

 peripheral circumference, and on their free projecting margins concentric 

 lines form themselves. It was at one time believed that a new lamella was 

 formed each year, and that the concentric lines were the thrown up projecting 

 margins of individual lamellae; for example, that a twelve-year-old fish had 

 twelve individual lamellae and twelve concentric lines corresponding to the 

 number of lanielhe. In a previous paper Iloffbauer showed that this accepta- 



* Hofn)auer, 1899. 



