INTRODUCTION 9 



optic ganglia and nerve-fibres with the brain. For a 



-m 





on 



Fig. 3. — A, Spermatozoon of a Longhorn Beetle (Morimus) : h, 

 head with nucleus ; w, middle-piece with centrosome ; /, flagellum. 

 B, Ovum or Egg of a Midge (Chironomus), pr, protoplasmic layer ; 

 y, yolk-globules ; on, egg-nucleus ; sn, sperm-nucleus (which has 

 entered egg) ; pn, polar nucleus, separated from egg-nucleus. X 400. 

 After Ballowitz and Ritter, Zeitschr.f. wissensch. Zool. 1, 1890. 



chemical stimulus to aff'ect nerve-cells so as to give rise to 

 sensations like smell or taste, it is necessary that the cuticle 



