DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
649 
ceding one, both in its structure and in the way in which it has been formed. It is 
the one, by various modifications of which are produced all the ridges and tubercles 
which ornament the surface of the scale : along the greater part of the vertical sec- 
tion fig. 2, it exhibits the appearance, which, when more highly magnified, is seen 
in fig. 3 c ; it has an undulating outline, and presents indistinct traces of a lamellar 
structure, the more external lamellae being parallel with the superior margin of the 
section. The surface of this portion of the scale, when viewed horizontally by means 
of reflected light, is seen to he studded with innumerable minute tubercles. 
When we approach the anterior margin of the scale (fig. 1 h), we find that the 
radiating lines already spoken of, are produced by the absence of the superficial 
tissue along their course, whilst in the intervening ridges it is of considerable thick- 
ness ; each ridge thus formed is transversely subdivided by very numerous minor 
ridges, the upper edge of each of which is sharp and crenulated. This structure will 
be readily understood by a reference to fig. 7? which represents a small portion cut 
out and viewed as an opake object ; 7 « are the lines along which the uppermost layer 
is wanting. These lines are homologous with those which M. Mandl regarded as nu- 
trient canals, an error which we have already seen to have been detected and in part 
corrected by M. Agassiz. But whilst the Swiss philosopher very properly pointed 
out their nature, he also erred in considering that they passed through the entire 
calcareous portion of the scale and reached the subjacent soft tissues. It is per- 
fectly true that they do so at the margin of the scale, where the middle layer is not 
yet developed, as seen in fig. 7 ; but towards the central portion of this and all ana- 
logous scales, where the middle layer exists, these grooves do not pass through it, ex- 
cept in some scales where at the anterior margin all the three tissues are cut through 
and the border of the scale is converted into a series of digitations, as is the case in 
that of the Perch : this however is merely an incidental circumstance, and does not 
affect the true nature of these grooves as they exist on the upper surface of the scale. 
The mistake made by M. Agassiz was one easily fallen into, he not being aware of 
the duplex character of the calcareous portion of the structure. 
Fig. 7 h represents the ridges of the upper layer with their transverse crenulated 
subdivisions. These latter are the true homologues of the concentric lines commonly 
seen on the surface of Cycloid and Ctenoid scales, but which in this instance are only 
developed on the surfaces of the radiating ridges ; 7 c are the extremities of the ridges 
as seen in the vertico-transverse section, whilst <xt 7 d we have an analogous vertical 
section, only made parallel to the superficial ridges, and in which the subjacent mem- 
branous laminse (7 e) are seen rising in succession towards the surface of the scale. 
These subdivisions of the superficial layer are the most conspicuous at the extreme 
anterior and lateral margins. As we approach the centre of the scale they become 
less definite, and are ultimately lost amongst the irregularly disposed superficial 
tubercles, of which the ridges are only a modified form. But at the posterior or free 
margin we find this upper layer assuming a new aspect. The superficial tubercles 
