126 SCIENCE PROGRESS. 



plates, one is tempted to hazard the suggestion that the 

 nervous discharge may actually pass through the chroma- 

 tophore from one nerve-plate to the other, and after the 

 manner of an electric discharge supply the agency necessary 

 to induce mutual repulsion or attraction on the part of the 

 pigment granules. 



In reptiles, Briicke's paper on the Chamaeleon (32) 

 remains classical. It was in this paper that Briicke showed 

 for the first time that during the so-called "contraction" of 

 the chromatophores in Vertebrata, the processes or branches 

 are not themselves withdrawn, but merely the pigment 

 within them. 



I have already drawn attention to Kerbert's observations 

 (33) on the development of the chromatophores in snake 

 embryos ; the branched pigmented cells appear in the epi- 

 dermis before they are recognisable in the mesoderm. But 

 Kerbert himself by no means holds that the chromatophores 

 are therefore of ectodermal origin. At the stage when pig- 

 ment first arises in the ectoderm he finds that both ectoderm 

 and mesoderm contain a number of round or oval cells en- 

 closing a central fluid vacuole ; and he interprets these cells 

 as the young stages of the chromatophores. He regards 

 them as connective tissue cells which penetrate into the epi- 

 dermis, develop pigment granules, and acquire a branched 

 form. The only serious objection that can be urged against 

 this view is that the author does not establish it. He shows 

 us the vacuolated cells in both layers, and the branched pig- 

 mented cells in the epidermis ; but he does not show us the 

 vacuolated cells becoming chromatophores, although he 

 makes the statement that here and there the vacuolated 

 cells may be seen to be obviously filled with pigment. 

 This is, however, not quite consistent with one of his 

 earlier statements to the effect that pigment first appears 

 in the ectoderm in the form of branched cells only slightly 

 pigmented, the granules being confined to the edges of 

 the cells, and occurring to a slight extent in the pro- 

 cesses. 



Kerbert finds it difficult to account for the appearance 

 of chromatophores in the ectoderm, since in the adult snake 



