RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 347 
secretion of the underlying cells passes, as is seen in Fig. 140, A and 
B. This cuticle corresponds to the chitinous covering of the 
arthropod, and like it is perforated with canaliculi, and, according to 
Lwoff, possibly contains chitin. The epidermal cells rest on a thick 
layer of most striking appearance (Fig. 141), for it resembles, in an 
extraordinary degree, when examined superficially, a layer of chitin ; 
it is called the laminated layer, and is characterized by the extreme 
regularity of the lamine. This appearance is due, as the observa- 
tions of Miss Alcock show, to alternate layers of connective tissue 
fibres arranged at right angles to each other, each fibre running a 
straight course and possessing its own nucleus. Although the fibres 
in each layer are packed close together, they are sufficiently apart 
to form with the fibres of the 
alternate layers a meshwork 
rather than a homogeneous 
structure, and thus the surface 
view of this layer shows a 
regular network of very fine 
spaces through which nerve- 
fibres and fluid pass. This 
layer is easily dissolved in a 
solution of hypochlorite of soda, 
a fluid which dissolves chitin, Fie. 140—Eprrtaunian CrLts or AmmMo- 
‘Any one Tsoking at Ammo: CO™® 2° eROw oem CANALICULT IN THE 
ny g Yuck Curicnz (B). A, TRANSVERSE 
ccetes would say that the only = Sxomion ruroveH THE CuriczE. 
part of its skin which résembles 
chitin is this laminated layer, and therefore the only part of its 
skin which would afford an indication of the nature of the 
skeleton of Pteraspis is this laminated layer, which belongs to 
the cutis, and not to the cuticle. Yet another significant peculiarity 
of this layer is its entire disappearance at transformation. Miss 
Alcock, in a research not yet published, has shown that this layer 
is completely broken up and absorbed at transformation; the cutis 
of Petromyzon is formed entirely anew, and no longer presents any 
regular laminated character, but resembles rather the sub-epidermal 
connective tissue layer of the skin of higher vertebrates. This 
laminated layer, then, just like the muco-cartilage, shows, by its 
complete disappearance at transformation, its ancestral character. 
Very suggestive is the arrangement of the different skeletal 
