82 EMBBYOLOGY OF THE LOWEE VEETEBEATES oh. 



heliotropic tendencies of the chromatophores which lead them to 

 migrate actively towards the illuminated side and there to remain. 



The chromatophores of Vertebrates often display their sensitive- 

 ness to light very markedly by movement reactions. Such are well 

 seen in the young stages of many fishes and Amphibians. In the 

 young Lepidosiren for example the chromatophores during the day 

 have their pseudopodia extended in all directions and their bodies 

 flattened out into a plate-like form so that they constitute a light- 

 proof coat giving a rich 

 purplish - black effect. At 

 dusk the pseudopodia become 

 slowly withdrawn so that a 

 few hours after darkness has 

 set in the chromatophores 

 have shrunk into minute 

 spheres so wide apart as to 

 have no influence on the 

 general colouring. The 

 young fish is then practically 

 colourless except for the 

 large yellow chromatophores 

 here and there which remain 

 expanded. 



During the course of 

 development in many fishes, 

 anurous Amphibians, and a 

 few Eeptiles such as the 

 chameleon, the compara- 

 tively simple reactions to 

 light such as have just been 

 indicated develop into reactions of a much more complex type in 

 which the central nervous system is involved. Eesearch into the 

 development of these more complex reactions is highly desirable for 

 at present little is known regarding them. 



Pig. 50.— Section through epidermis of Lepidosiren 

 larvae. 



A, fixed under conditions of dull daylight ; B, under 

 conditions of darkness during the night. (The chroma- 

 tophores in the dermis, which crowd together under the 

 inner surface of the epidermis, are not shown in A.) 

 p, chromatophore ; v, blood-vessel. 



NERVOUS SYSTEM 



The nervous system, which has to do with the receiving' of, and 

 the reacting towards, impressions from the outer world, appears to 

 have arisen in evolution, as might have been expected, from the 

 outer layer or ectoderm. The first steps in the evolution of the 

 Vertebrate nervous system are not within the scope of direct obser- 

 vation but the view is probably correct that it arose from a diffuse 

 subepidermal network or plexus of the type still persisting in some 

 of the more primitive invertebrates. In the development of the 

 Vertebrate embryo the main parts of the nervous system may still be 

 seen to take their origin from the ectoderm. 



