6 



ARKIV FÖR ZOOLOGI. BAND 8. N:0 10. 



arise from it. The tips of the spines penetrate the epider- 

 mis. As is shown in diagram A the exteriör layer of the 

 corium forms numerous projections into the epidermis, one 

 for each branch of the spine. Through the development of a 

 distinct basal membrane and especially thanks to the exi- 

 stence of pigment cells in the corium along this membrane, 

 the relation of the epidermis to the spines is easy to trace. 

 The epidermis does not touch the spines except just at their 



Fig. C. Mola. Diagram showing the growth of a scale. The lines repre- 



sent the homogeneous part, the points the fibrillated part. a is the part 



first formed which consists solely of homogeneous substance. 



tips, which project — though to a very slight extent — above 

 the surface of the skin. This fact is of great interest as 

 regards the problem of the formation of the spines and will 

 be dealt with below. Turner writes about the plates as 

 follows (p. 187): »The tubercles themselves consisted of two 

 parts, which possessed distinctive microscopic characters, a 



superficial cuticular and a deeper härd portion, The 



cuticle covered the superficial surface of tubercles and ex- 

 tended for some distance up the sides of the spines, although 

 I never saw it passing över their free ends. It was conti- 



