CLIMAX OF THE ANIMAL KINGDOM— PHYLUM CHORDATA 301 



during the animal's life. The last part of the statement is necessary be- 

 cause there are a few chordates that have dispensed with their tails in 

 the adult stage. These include frogs, guinea pigs, apes, and man. Yet, 

 all of these, including man, have tails in their early development, but they 



Photo by C. F. Reather, Carnegie Inst, oj Washington, Dept. of Interior 



Fig. 21.1. A human embryo about 32 days of age showing the gill clefts and tail. 

 These are chordate characteristics shown by all chordates in their early embryonic life 

 and which persist into the adult life of the lower chordates. Note the limb buds, not 

 yet differentiated into fingers and toes ; the heart region which must be very large to 

 accommodate the comparatively large heart ; the muscle segments down the back ; and 



the umbilical cord. 



fail to develop in proportion to the rest of the body and are rudimentary 

 in the adult animals. Man has a tail bone, consisting of several small 

 vertebrae fused together, as a remnant of his embryonic tail. Occasion- 



