ORGANIC EVOLUTION 45 



into organs of nutrition and reproduction, and by 

 the development of the ectoderm, which is the outer 

 cell layer, into organs of motion and sensation. 



The embryonic development of a human being 

 is not different in kind from the embryonic de- 

 velopment of any other animal. Every human 

 being at the beginning of his organic existence 

 is a protozoan, about y^t ^^ch. in diameter ; at 

 another stage of development he is a tiny sac- 

 shaped mass of cells without blood or nerves, the 

 gastrula ; at another stage he is a worm, with a 

 pulsating tube instead of a heart, and without 

 head, neck, spinal column, or limbs; at another 

 stage he has, as a backbone, a rod of cartilage 

 extending along the back, and a faint nerve cord, 

 as in amphioxus, the lowest of the vertebrates ; at 

 another stage he is a fish with a two-cha.mbered 

 heart, mesonephric kidneys, and gill-slits with gill 

 arteries leading to them, just as in fishes ; at 

 another stage he is a reptile with a three-chambered 

 heart, and voiding his excreta through a cloaca like 

 other reptiles ; and finally, when he enters upon 

 post-natal sins and actualities, he is a sprawling, 

 squalling, unreasoning quadruped. The human 

 larva from the fifth to the seventh month of 

 development is covered with a thick growth of hair 

 and has a true caudal appendage, like the monkey. 

 At this stage the embryo has in all thirty-eight 

 vertebrae, nine of which are caudal, and the great 

 toe extends at right angles to the other toes, and is 

 not longer than the other toes, but shorter, as in 

 the ape. 



