THE ANIMAL ANCESTRY OF MAN 



59 



and opposing groups. By means of the already bewildering 

 connection systems of the central nervous system, stimula- 

 tions of the primitive sense organs (representing the senses 



B 



ecfoderm. 

 nerirefube 



mesoderm 

 noiochorc( 



enfoder?n 



ecfoderm 

 nerireiube 

 mesoder?72 

 notochorcC 



entoderm. 



Fig. 2. Lancelet (Amphioxus), lowest existing chordate. 



A. Diagram of anatomy of adult animal. 



(After Marshall and Hurst, Practical Zoology.) 



B. Section of embryo, showing relations of mesoderm and entoderm. 



(After Cerfontaine, from Shumway's Vertebrate Embryology, John Wiley & Sons.) 



c. Cross-sections of three stages of development in frog, showing relations 

 of mesoderm to outer and inner germ layers. 



(After Hyman, Laboratory Manual for Comparative Vertebrate Anatomy, Chicago 

 University Press.) 



of feehng, taste, smell, sight, balance, bodily position, etc.) 

 could be organized and directed toward successful motor 

 responses. All this matter hes within the special field of the 

 neurologist and is fully dealt with elsewhere in this book 

 (Chaps. Ill, xi). The present chapter is concerned primarily 

 with the muscular and skeletal elements of the vertebrate 

 locomotor apparatus rather than with the nervous mech- 

 anism of direction and control. 



At a very early period the primitive muscle pouches 

 became transformed into zigzag-shaped muscle segments. 



