102
P. W. KENT
deposition of cuticle (15). In Palinurus (67), in segments of blow fly
puparium (68), and in the tarantula leg, crystallites are oriented with
respect to a morphological axis, in the latter case being parallel to the
direction of maximum strength.
In the peritrophic membrane of insects, the lamellae are constructed
(69) of triplet fibrils inclined at 60 degrees to each other. Picken (J5)
has pointed out that this favors the idea that biosynthesis of chitin chains
and their precipitation is an extracellular process, perhaps from a proteinpolysaccharide complex (70).
Elsewhere it appears possible that isotropic chitin is a statistical effect
either at the molecular level or through variations in successive lamellae
(71-73).
In addition, other processes are known for the cross-linking of proteinaceous structures, particularly in hard chitin, involving condensation of
reactive groups on adjacent macromolecules by "tanning agents" such as
o-quinones. These processes of sclerotization have been reviewed elsewhere (13, 43).
To this must be related a second biological process of importance
in the formation of hard chitin by calcification. Practically all cuticles
contain inorganic constituents ranging between trace quantities and 99%,
chiefly in the form of calcium salts though magnesium, iron, aluminum,
and silicon have also been detected (74). The commonest crystalline
deposits (75-79) are forms of calcium carbonate, particularly aragonite.
This occurs in mollusk shells whereas arthropods seldom if ever contain
this modification. Calette is commonly reported and occurs in arthropods
as extremely thin plates or granular aggregates, frequently with different
orientations in adjacent areas. The resulting effect is to produce a mosaiclike structure. The variation in orientations of crystal plates has been
ascribed to the initiation of crystal lattices at different faces, crystallization continuing until impeded by adjacent plate boundaries.
In principle, the calcification of cuticle resembles that of bone in
higher animals; i.e., it is a discreet biochemical process in which the
mineral deposits do not reflect the composition of the ionic substances
of blood nor, in this case, that of a possible external aqueous environment. Furthermore, as in bone, calcification involves mineral deposition
in a preformed organic matrix. In cuticle, the laminar structure is, however, characteristic. A general trend in which the extent of calcification is
inversely related to the amount of protein present in cuticle is discerned
from the work of Lafon (80). Hence during the course of calcification,
the proportion of protein to chitin decreases markedly (81). In some
species, e.g., Gammarus, the calcification occurs rapidly (82) and is
P. W. KENT
deposition of cuticle (15). In Palinurus (67), in segments of blow fly
puparium (68), and in the tarantula leg, crystallites are oriented with
respect to a morphological axis, in the latter case being parallel to the
direction of maximum strength.
In the peritrophic membrane of insects, the lamellae are constructed
(69) of triplet fibrils inclined at 60 degrees to each other. Picken (J5)
has pointed out that this favors the idea that biosynthesis of chitin chains
and their precipitation is an extracellular process, perhaps from a proteinpolysaccharide complex (70).
Elsewhere it appears possible that isotropic chitin is a statistical effect
either at the molecular level or through variations in successive lamellae
(71-73).
In addition, other processes are known for the cross-linking of proteinaceous structures, particularly in hard chitin, involving condensation of
reactive groups on adjacent macromolecules by "tanning agents" such as
o-quinones. These processes of sclerotization have been reviewed elsewhere (13, 43).
To this must be related a second biological process of importance
in the formation of hard chitin by calcification. Practically all cuticles
contain inorganic constituents ranging between trace quantities and 99%,
chiefly in the form of calcium salts though magnesium, iron, aluminum,
and silicon have also been detected (74). The commonest crystalline
deposits (75-79) are forms of calcium carbonate, particularly aragonite.
This occurs in mollusk shells whereas arthropods seldom if ever contain
this modification. Calette is commonly reported and occurs in arthropods
as extremely thin plates or granular aggregates, frequently with different
orientations in adjacent areas. The resulting effect is to produce a mosaiclike structure. The variation in orientations of crystal plates has been
ascribed to the initiation of crystal lattices at different faces, crystallization continuing until impeded by adjacent plate boundaries.
In principle, the calcification of cuticle resembles that of bone in
higher animals; i.e., it is a discreet biochemical process in which the
mineral deposits do not reflect the composition of the ionic substances
of blood nor, in this case, that of a possible external aqueous environment. Furthermore, as in bone, calcification involves mineral deposition
in a preformed organic matrix. In cuticle, the laminar structure is, however, characteristic. A general trend in which the extent of calcification is
inversely related to the amount of protein present in cuticle is discerned
from the work of Lafon (80). Hence during the course of calcification,
the proportion of protein to chitin decreases markedly (81). In some
species, e.g., Gammarus, the calcification occurs rapidly (82) and is