76 



AN INTRODUCTION TO ZOOLOGY. 



scope, the thin embryonic layers already spoken of 

 are seen one below another (Fig. 23). In the middle is 

 seen a round body which is a section of a hollow tube ; 

 in very early stages this round body is seen to be formed 

 at first as a long median fold of the outside layer, 

 from which it is afterwards constricted off, so as to 



Fig. 20. — !N"otoch.ord (highly magnified) of embryo duck in transverse sec- 

 tion. At duck of four days' incubation; the cells have large oval nuclei, often 

 containing two nucleoli (p. 20), and contain large vacuoles (p. 22). iJ, duck 

 of six days' incubation; the vacuoles are much larger, so that the cells are re- 

 duced to mere threads lying between them. (For position of notochord see 

 Fig. 23, p. 113.) 



form a closed tube lying inside (Fig. 24). This closed 

 tube afterwards becomes the brain and spinal cord, or 

 central nervous system of the body, the bony protec- 

 tion of which we call the skull and spine. The fact 

 that the spinal cord is thus formed shows us that the 

 nerve tissues are formed by modification of the skin 

 layer. (See Chap. III.) Underneath this spinal cord 



