BRANCH CHORDATA 189 



Characteristics. They all have (1) Gill-slits in their embry- 



Fig. 151. A series of embryos at three comparable and progressive 

 stages of development (marked I, II, III) , representing four different di- 

 visions of the class Mammalia. (After Hackel.) 



onic life or they may be permanent; (2) a notochord; (3) a nerve 

 chord or nervous system dorsal to the notochord. The noto- 



