THE CHROMATOPHORES OF ANIMALS. 115 



ectodermal cells beneath the general layer of the epithe- 

 lium. 



Finally, Kerbert's (3$) interesting" observations on 

 Vertebrata have shown that during the development of 

 reptiles the chromatophores appear first in the epidermis, 

 and only subsequently in the cutis. In the Lacertina^ a 

 certain number of chromatophores are retained in the epi- 

 dermis even in the adult ; but in snakes, whose scales attain 

 a higher decree of differentiation than is the case with those 

 of lizards, no chromatophores are retained in the epidermis 

 after embryonic life ; the epidermic stage of the chromato- 

 phores in snakes is transitory. 



It would thus appear that there is a considerable amount 

 of evidence in favour of the view that chromatophores in 

 general are of ectodermal origin, whatever be their ultimate 

 position in the body. 



The Chromatophores of Crustacea. The literature 

 which bears upon the subject of chromatophores in Crustacea 

 deals chiefly with the sessile-eyed Malacostraca, and the 

 investigations which point to the ectodermal origin of 

 chromatophores deal principally with certain Isopoda. 



In Trichoniscus, according to Max Weber (1), the ecto- 

 derm is a syncytium and consists of nuclei embedded in a 

 granular sheet of protoplasm beneath the chitinous cuticle. 

 Only here and there, for example at the junction of two 

 segments, can a definite epithelium be distinguished. The 

 chromatophores lie partly in the connective tissue of the 

 body cavity, and partly in a layer just beneath the ectoderm. 

 In the latter case, however, although the bodies of the 

 chromatophores may lie beneath the ectoderm, many of 

 their processes are found within the syncytium itself and 

 often extend as far as the under surface of the cuticle. 

 Here and there indeed entire chromatophores are found 

 embedded in the subcuticular syncytium, and their branches, 

 anastomosing with those of neighbouring chromatophores, 

 form a net-work within the syncytium, which often pro- 

 duces the appearance of a regular epithelium, especially 

 when the meshes of the net-work happen each to enclose 

 one of the nuclei of the syncytium. In Trichoniscus 



