10 



The dermis is formed of four ill-defined layers; an outer layer of fibrous 

 connective tissue, a sheet of chroraatophores, a deep fibrous layer and a layer 

 of iridiocysts. The superficial fibrous layer supports the epidermis and has 

 numerous nerve fibres in its deeper portion. 



The second layer is characterized by the remarkably specialized chromato- 

 phores. Each chromatophore consists of a pigment cell, its capsule of elastic 

 tissue, and a circle of radiating muscle cells. The chromatophores have been a 

 subject of lively interest and protracted controversy' and innumerable papers upon 

 their anatom>', ontogeny and physiology have appeared. We have not indulged 

 the natural desire to enter upon an extended investigation and discussion of such 

 beautiful , attractive and much studied objects. We are, however, in 'essential 

 agreement v\'ith Chun's results. The pigment cell and muscle cells of each 

 chromatophore arise from a single mesodermal cell. The centi'al cell secretes a 

 granular pigment which in our species is of a dark reddish brown color in the 

 larger and more numerous chromatophores and yellow in the others. The 

 pigment is laid down in a fluid within a rather sharply marked cavity 

 and the nucleus is gradually forced to the periphery and usually lies beneath 

 the pigment mass. The yellow pigment is a lipochrome , the brown pigment 

 has been found by Dr. Alsberg of the Harvard Medical School, to be melanin 

 and is consequently essentially like the ink. The nucleus is large, takes deep 

 colors with chi'omatic stains, and has a large nucleolus. 



The capsule is thin, elastic, and is formed by a number of cells, a part or 

 all of which probably arise from the original chromatophoric cell. The flattened, 

 lightly stained nuclei are easily distinguishable from those of the pigment and 

 muscle cells. The elasticity of this sheath keeps the pigment mass spherical 

 except during the contraction of the muscle cells. 



The nuclei of the cells which are to become muscle cells group themselves 

 in an irregular circle around the pigment cell. Each cell sends out a long 

 tapering process which extends into the loose connective tissue of the dermis 

 and is attached either by branches which are attached to the connective tissue, 

 or fixed by anastomosis with other muscle fibres. The broad central end of the 

 muscle cell, — or perhaps we should say of a muscular portion of the chromato- 

 phoric syncytium , since the boundaries of pigment , sheath and muscle cells are 

 at least indistinct if not absent , — contains the elliptical or oval nucleus 

 which is surrounded by a small amount of granular protoplasm. The remainder 

 of the protoplasm is fibrillar. The contraction of the muscle cell, producing 

 as it does a large expansion of the circumference of the pigment cell , necessarily 

 broadens its own base, and the nucleus takes a position perpendicular to the 



