52 HOW ANIMALS DEVELOP 



narrow ridge across about three-quarters of the area. 

 It takes about eighteen hours to grow to its full 

 length, which is about 3 mm., so the rate of elonga- 

 tion is not excessively fast, only about a yard a month. 

 The mesoderm is produced from this ectodermal 

 thickening or primitive streak, first in its front end 

 and gradually further and further back. When 

 mesoderm production begins the tissue at the sides 



ECTODERM 



MESODERM 



ENDODERM 



Fig. 13. — Section across the primitive streak of the chick 

 showing the origin of the mesoderm 



moves straight across the transparent area towards 

 the primitive streak, dives down into it from each 

 side, turns round, and grows out towards the sides 

 again between the ectoderm and the endoderm 

 (Fig. 13). If a little finely ground Indian ink is put 

 on to the surface of the primitive streak and the 

 embryo grown for a few hours in a culture vessel, 

 sections will show how the Indian ink has been 

 picked up by cells which originally lay on the surface 

 and has been carried down into the mesoderm. 



Finally, one more movement happens. The 

 streaming in towards the primitive streak ceases, 

 first in the front end, and instead a strong backward 



