106 SCIENCE PROGRESS. 



more immediate object of the present paper, namely, to 

 reveal what light has been thrown by increase of research 

 upon the evolution and mechanism of true chromatophores. 



Since it is evident that chromatophores are simply pig- 

 mented cells specialised for a particular purpose, the problem 

 of their evolution cannot be adequately considered except 

 by the comparative method, which in this case involves a 

 survey of pigment-cells in general. 



In this survey special attention has been paid to two 

 points : — ■ 



i. The localisation of the pigment-cells and their rela- 

 tion to the germinal layers. 



ii. The nature of the structural modifications exhibited 

 by pigment-cells. 



I have neglected all references to intercellular pigmenta- 

 tion, since this phenomenon, important as it is in regard to 

 the history of pigment-cells in general, has no particular 

 bearings upon the origin, of chromatophores. 



i. Porifera. Pigment-cells of oval, spindle-shaped, or 

 stellate forms are described by Schulze and Vosmaer for 

 various non- calcareous sponges. Their origin appears to 

 be unknown (Vosmaer, Bronns Klasseu unci Ordnungen, 

 ii., 1887). 



2. Ccelentera. In the Hydrozoa pigment may be de- 

 posited either in the ectoderm or in the endoderm. The 

 varied colours of the Hydroidea, especially the Gymno- 

 blastea, are almost entirely due to pigmentation of the 

 endodermal cells (Allman), but ectodermal pigment is found 

 in the ocelli of Craspedote Medusae and occasionally in the 

 epithelium covering the generative glands (Hertwig). 



The brilliant colours of many Acraspedote Medusae 

 appear to be chiefly due to pigments deposited in the 

 ectoderm (Haeckel). 



In the Anthozoa the brilliant colours are chiefly due 

 to so-called zooxanthellae, which have been interpreted by 

 the Hertwigs as symbiotic algae. The earlier observers 

 (Haime, M. Edwards, Gosse) described a special pig- 

 mented layer of the ectoderm ; but H eider has failed 

 to find any special pigment-cells, and he attributes the 



