BRANCHIAL CARTILAGES OF LIMULUS. 3 II 



outside this layer there may be a very sharp, thin, blue band, or a broad red band 

 of a lighter color. The most intense and widest blue bands form the middle 

 layer in the partitions between two cells, or between two groups of cells, or in the 

 layers surrounding the whole nest. 



In some instances, after staining a short time only, these reactions seem to 

 be reversed, the violet bands appearing blue and the blue ones violet. The 

 violet bands are the first to stain, the blue ones appearing much later; they show 

 most clearly after the sections have been partially decolorized in glycerine. 

 Similar color bands are seen after staining with heematoxylin, except that the 

 bands are of different shades of purple. 



If thionin sections are treated with weak picric acid, the blue bands become 

 intense green, and later bright yellow, while the violet bands are affected but little. 

 These preparations are most brilliant and are the ones from which the drawing 

 was made. 



In the axial portions of the gill bars, the cell nests in some cases, are sepa- 

 rated by considerable areas of a nearly homogeneous matrix, some parts being 

 blue, others violet. The perichondrium stains a bright characteristic violet, 

 and the muscles blue. 



The probable explanation of these complicated color reactions is that the 

 walls of each capsule consist of a graded series of different chemical substances 

 arranged in concentric layers, the substance giving the red reaction being the 

 most abundant in the inner protoplasmic layers of the capsules, and that giving 

 the blue reaction being more abundant on the periphery, with intermediate 

 transitional compounds between; and that the blue material is formed by a gradual 

 modification of the red. 



The gill bars of Limulus are exceedingly interesting histologically, and deserve 

 a more careful and detailed description than we can give them here. The main 

 points we desire to establish now are that they contain true cartilage of a very 

 primitive type, and that this cartilage is quite different, chemically and histologi- 

 cally, from that found elsewhere in Limulus. 



The reaction of the gill cartilages to thionin and haematoxylin indicates that 

 they contain, besides other substances, a considerable amount of mucin. This 

 fact, together with their remarkable histological structure, emphasizes still more 

 strongly the resemblance between the gill bars of Limulus and those of Petro- 

 myzon. Moreover, the extraordinary difference between the capsuligenous 

 cartilage and the fibro-cartilage of the endocranium in Limulus is paralleled by 

 the fact that there is a corresponding difference between gill cartilage and cranial 

 cartilage in the cyclostomes. 



The fibro-cartilage of the arachnids appears to have arisen by the gradual 

 transformation of fixed muscle ends into sinews or tendons, in proportion as the 

 corresponding muscles become more active and voluminous. The capsuligenous 

 cartilage appears to arise "de novo," making its appearance as an entirely 

 different looking material from that in which it is imbedded. The gill bars 



