PIGMENT TISSUES 



271 



The origin of segregated pigment in the vertebrates has been much 

 discussed, and it is not yet settled. In the higher invertebrates there 

 is little doubt that dark pigment granules may be elaborated by the 

 epidermal cells as well as by connective-tissue cells. Sections of the 

 mantle epidermis of the mussel, Mytillus, for instance, show pigment 

 granules that have quite likely been elaborated by the epidermal cells 

 under normal conditions. Schiedt has shown definitely that dark pig- 

 ment granules develop under pathological or abnormal conditions within 

 the epidermal cells of the oyster. But even among the Invertebrata 

 the connective-tissue cells are the chief elaborators of pigment granules. 

 These cells assume various shapes and, in some cases, become very 

 highly specialized and are under the control of special nervous impulses. 

 Such is the case with the large pigment cells or chromatophores of the 

 cephalopod mollusks. In the mammals, or at least in man, melanin 

 granules are a connective-tissue product. Ehrmann, in a recent article, 

 advances the following views concerning this conspicuous pigment: 

 "Melanin is intra-cellular, and in the situations where it is present it 

 occurs in the deeper lay- 

 ers of epidermal cells 

 and in certain meso- 

 blastic cells known as 

 melanoblasts.' The me- 

 lanoblasts are special- 

 ized connective-tissue 

 cells which are round, 

 spindle-shaped, or 

 branching, and are 

 peculiar not only for 

 containing melanin 

 granules, but also for 

 having larger nuclei 

 which stain more faintly 

 than those of ordinary 



cells. Melanoblasts occur in the upper layers of the corium, are espe- 

 cially noticeable around the blood vessels, and are also present as peculiar 

 structures in the interepithelial lymph-spaces of deeper portions of the 

 epidermis. The substance is a derivative of blood-pigment, the mate- 

 rial of which it is formed getting out of the bipod vessels into the peri- 

 vascular tissue spaces, where it is taken up by the melanoblasts and 

 transformed into melanin. The epidermal cells do not elaborate 

 melanin, but absorb it from the melanoblasts in the interepithelial 

 lymphatics." 



The connective-tissue cells that have become highly specialized as 



FIG. 234. Two figures of dividing pigment cells from the 

 skin of a larval salamander. (After ZIMMERMANN.) 



