August 21, 1914] 



SCIENCE 



281 



tion under the influence of light were very 

 marked and never failed as long as the animals 

 remained alive, while dead tissue generally 

 disintegrates without change in pigmentation, 

 due to bacterial fermentation. Gortner has 

 proved that, in the case of the cicada, the 

 oxidase is secreted together with the new 

 cuticula, which ceases to be formed in the 

 absence of life processes, for, when he washed 

 and rubbed dead adults in a stream of water 

 coloration took place only in spots, mostly in 

 the folds of the abdomen, where the cuticula 

 had not been completely removed, while othei 

 adults which had been as thoroughly washed 

 but not killed, slowly darkened to the normal 

 color. We may have a similar condition exist- 

 ing in the living oyster in which the regenera- 

 tion of the shell, the homologue of the cuticula, 

 takes place. In the case of the oyster, however, 

 light must play the chief role, for when placed 

 in darkness the depigmentation takes place in 

 the epidermis of the oyster while the shell 

 continues to grow, and, besides, pigmentation 

 always takes place over the gills, the epidermal" 

 cells of which do not form a shell. Faussek's 

 negative results with light can only be ex- 

 plained on the basis of the additional factor 

 of temperature and perhaps of food conditions. 

 If the oysters were placed so far below the 

 surface of the water that the refraction caused 

 a decomposition of the light and a consequent 

 lowering of temperature or maximum energy 

 and with it a diminution in the food supply 

 the conditions for pigmentation became un- 

 favorable and the results negative. On the 

 other hand, Gortner's observations on the non- 

 interference of light with process of pigmen- 

 tation can be explained on the basis of the 

 normal occurrence of maximum pigmentation 

 in the cicada under any light condition as a 

 hereditary factor, while in the oyster the 

 epidermis of the mantle with the exception of 

 the mantle edge is colorless when covered by 

 the shell: the mantle edge is pigmented be- 

 cause it is generally free from the shell and 

 exposed to light. In the one case contact with 

 the atmosphere through the cuticula is essen- 

 tial, in the other light and temperature, for we 

 find that oysters at certain depth or when 



covered with mud are lacking the pigment 

 even in the mantle edge. 



So much for the purely physico-chemical 

 sources of pigmentation. Considering the 

 biological side of pigmentation, a study of the 

 structure of pigments and pigment cells is 

 essential. There are different views on this 

 subject. It is, however, generally agreed that 

 pigments occur both as excretions and secre- 

 tions either in a difEused or in a segregated 

 form; in the one case they may be masses of 

 colored granules represented by bilirubin, 

 uroohrome, melanin, etc., in the other they 

 may be variously shaped cells, called chroma- 

 tophores or melanoblasts. Keller distinguishes 

 in the chamseleon melanophores, leuco- 

 phores, ochropores, xanthophores and erythro- 

 phores, while Kromayer argues that pigment 

 cells are not cells, but epithelial figures which, 

 however, can not be isolated. Eeinke again 

 distinguishes between pigment carriers and 

 pigment substance, the latter consisting of 

 colorless prisms, scales, granules, etc. Ehr- 

 mann holds that " melanin is intra-cellular, 

 and in the situations where it is present it 

 occurs in the deeper layers of epidermal cells 

 and in certain mesoblastic cells known as 

 melanoblasts. The melanoblasts 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 especially noticeable around the 

 blood vessels and are also present as peculiar 

 structures in the interepithelial lymph-spaces 

 of deeper portions of the epidermis. The sub- 

 stance is a derivative of blood pigment, the 

 material of which it is formed getting out of 

 the blood vessels into the perivascular tissue 

 spaces, where it is taken up by the melano- 

 blasts and transformed into melanin. The 

 epidermal cells do not elaborate melanin, but 

 absorb it from the melanoblasts in the inter- 

 epithelial lymphatics." The most recent 

 views on the nature and origin of pigment in 

 the sebaceous glands of certain Cavicomians 

 have been advanced by Weber, Beccari and 



