88 EVOLUTION 



the higher embryo, and we will pass to more obvious 

 indications. 



In the third week of development, when the embryo 

 (less than a quarter of an inch long) has something of the 

 appearance of a tadpole (minus the gills) a large head 

 and long tail, with no trace of limbs a series of five 

 slits or folds appears in the throat or chest region. That 

 these (though not open) are real gill slits is seen at once 

 on dissecting the embryo. The rudimentary heart is 

 found to be in the position and of the same structure as 

 that of the fish, and its chief arteries rise in six double 

 arches over the gill arches. The whole of this distinc- 

 tively "fish" arrangement which is found also in the 

 embryos of all other mammals, reptiles, and birds has no 

 function and no utility. It will entirely disappear within 

 a week or two. It is a most striking illustration of the 

 mysterious law of the reproduction of ancestral stages, 

 and a most curious reminiscence of the Silurian fish 

 ancestor of all the vertebrates. Nor is it the only clear 

 indication of our fish ancestry. The nose makes its 

 appearance in the fourth week as a pair of simple 

 depressions in the skin, just as we find the organ of 

 smell appearing in worm-like animals below the fish 

 stage. The pits then connect with the mouth by an open 

 groove (as in the shark): the grooves are closed over 

 and become nostrils leading into the front of the mouth 

 (as in the Dipnoi): and the successive development passes 

 through the reptile and early mammal stage. The 

 external prominence does not form until about the 

 tenth week. The jaws, ears, limbs, heart, diaphragm, 

 etc., show an equally interesting development. Before 

 birth the whole body is covered with a coat of fine 

 hair (like that of the ape) ; afterbirth the fingers and toes 

 show a remarkable power of grasping (like those of the 

 ape), and the spine is curved for a long period, so that 



