132 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tine identical in structure and development with those of the crown. 

 The root is likewise surrounded by a layer of bony tissue, whicli 

 develops like that of the maxilla at the expense of inter-dento-maxillary 

 connective tissue. Thus arises the osseous cortex (" cement " of some 

 authorities), while the rest of the inter-dento-maxillary tissue uniting the 

 cement to the maxilla persists in a fibrous or ligamentar state. 



J. A. T. 



Melanophores of Horned Toad. — Alfred G. Redfield (Journ, 

 Exper. ZooL, 1918, 26, 275-338, 5 pis., 8 figs.). In Phrynosomci 

 cornutum light produces an expansion, its absence a contraction of the 

 melanophore pigment. High temperatures produce contraction and low 

 temperatures expansion. The heat effect dominates at extremes of tem- 

 perature, the light effect at mean temperatures. Light coming from a 

 dark substratum produces an expansion, from a light substratum a 

 contraction of the pigment. Mild mechanical stimuli do not affect the 

 melanophores ; mild faradic stimuli cause contraction of the pigment, 

 and so do noxious stimuli, such as violent mechanical or faradic stimuli. 



The melanophores, or some closely associated tissues, are receptors of 

 photic and thermal stimuli. There are no specific receptors for noxious 

 stimuli. The eyes are receptors for stimuli which cause adaptive re- 

 actions of the melanophores. 



The melanophore pigment is contracted by the direct action of 

 nerves as well as by the direct action of adrenin. The spinal cord 

 contains, between the eighth and thirteenth vertebrae, nervous structures 

 through which pass the impulses which cause the contraction of the 

 melanophore pigment. Impulses pass from this part of the cord directly 

 to the adrenal glands. Impulses also pass from this part of the cord 

 posteriorly, and perhaps anteriorly, within the cord to segmentally 

 arranged peripheral nerves which connect directly with the melanophores. 

 The peripheral fibres are a part of the sympathetic division of the auto- 

 nomdc nervous system. 



The reactions of the melanophores of the horned toad produce a 

 series of colour changes correlated with tlie rhythm of day and night, an 

 adaptation of the colour of the skin to that of the environment, and a 

 characteristic pale condition of the skin during nervous excitement. The 

 daily rhythm of colour change is caused by the direct action of photic 

 and thermal stimuli upon the melanophores or some closely associated 

 tissue. The adaptive reactions of the melanophores depend upon stimuli 

 received through the eyes. The contraction of the melanophore pig- 

 ment during nervous excitement is brought about by the co-operation of 

 nervous impulses delivered to the pigment cells by the sympathetic 

 nervous system and the secretion of adrenin by the adrenal glands. 

 There is a marked resemblance between the innervation of melanophores 

 and the innervation of smooth muscle. J. A. T. 



Division of Melanophores of Trout. — P. Murisier {Bull. Soc. 

 Vaudoise, 1919, 52, 97-8). In Trutta lacustris the melanophores are 

 all mononuclear until a few days before hatching. The transition from 

 the mononuclear to the binuclear phase is accomplished by a typical 



