298 THE DERMAL SKELETON. 



innumerable canaliculas (pore canals of authors). They are extremely minute and 

 extend in straight lines or in finely wound spirals almost to the outer surface, p.c. 

 The most superficial layer, e, is thin, colorless, vitreous, and devoid of canaliculi. 

 Although very hard and polished, it is easily destroyed under some conditions. 

 In surface views it is seen to be divided by shallow grooves into polygonal facets 

 or zigzag ridges. (Fig. 200; D.) 



2. The middle layer consists of broad cancellous spaces separated by ir- 

 regular vertical partitions. The cancellae are filled with loose connective tissue, 

 through which ramify nerves and blood-vessels. 



3. The third, or inner, layer is composed of trabeculae arranged parallel to 

 the outer surface. It is horizontally laminated, and is pierced with large irregular 

 openings, through which nerves and blood-vessels pass to the cancellated layer. 



Each bar, or trabecula, is covered by a thin layer of pigmented ectoderm 

 continuous with that underlying the outer layer, and that secretes, or produces 

 by the periodic transformation of its own substance, the chitenous lamellae of the 

 shell. The lamellae are generally grouped in bands which differ in their color 

 and in their chemical reaction. The older, outer layers of the shield are dark 

 brown, and a band of this colored chiten extends into the axis of each trabecula. 

 The deeper lamellae of the outer layer, and the outer lamellae of the trabeculae, 

 are transparent and nearly colorless. 



The axial core of the trabeculae stains deeply in haematoxylin and in acetic 

 acid carmine, the outer layers remaining colorless. This fact is important, as it 

 indicates some preliminary chemical change in the axis of the bars, where the 

 bone cells later appear. 



In the oldest crabs, the axis of each bar is densely crowded with spindle- 

 shaped cavities, or lacunas. Their long axes are parallel to the long axis of the 

 bar, and, under favorable conditions, we can see that many of them are connected 

 at one end with a very fine tubule, or canaliculus, which runs radially toward the 

 periphery. The largest lacunae are nearest the center of the trabeculae. (Fig. 206,. 4.) 



If a section of the bone is dried in the air and then mounted in balsam or 

 glycerine, the lacunae and canaliculi appear black in transmitted light, and 

 silvery-white in reflected light, showing that by the drying up of their semi-fluid 

 contents they have become filled with air. Old fragments of bone that have 

 dried in the sun for an indefinite period, when softened and sectioned, show the 

 same structure. Caustic potash dissolves the granular contents of the lacunae, 

 but does not otherwise affect them. 



Stained sections, quickly transferred to balsam, have many of the lacunae 

 and canaliculi injected with the stain, which generally disappears when the sec- 

 tions are well washed. But even after long washing with acidulated alcohol, 

 some of the larger lacunas show a characteristic color due to the presence of a 

 faintly stained granular substance, and a small darker colored body. 



All these facts show that we are dealing with actual cavities and canals in 

 the chiten, some of which are filled with nucleated protoplasm. 



