CHAP. XI The Elements of Structure 179 



animals degenerate in eyes and fore -limbs respectively. 

 {c) But somewhat different are such structures as the 

 following : The embryonic gill-clefts of reptiles, birds, and 

 mammals, which have no respiratory significance, or the 

 embryonic teeth of whalebone whales, of some parrots and 

 turtles, which in no case come to anything. They are 

 vestigial structures, which are partly explained on the 

 assumption, justified also in other ways, that the ancestors 

 of reptiles, birds, and mammals used the gill-clefts as fishes 

 and tadpoles do, that the ancestors of whalebone whales, 

 birds, and turtles had functional teeth. No one can say 

 with certainty of vestigial structures that they are entirely 

 useless, nor can one precisely say why they persist after 

 their original usefulness has ceased. They remain because 

 of necessities of growth of which we are ignorant, and 

 they may be usefiil in relation to other structures though 

 in themselves functionless. 



Classification of Organs. — We may arrange organs 

 according to their work, some, such as limbs and weapons, 

 being busied with the external relations of the organism ; 

 others, such as heart and liver, being concerned with 

 internal affairs. Or we may classify them according to 

 their development from the outer, middle, or inner layer of 

 the embryo. Thus brain and sense-organs are always mainly 

 due to the outer stratum (ectoderm or epiblast), muscles 

 and skeleton arise from the middle mesoderm or mesoblast, 

 the gut and its outgrowths such as lungs and liver primarily 

 originate from the inner endoderm or hypoblast. Or we 

 may arrange the various structures more or less arbitrarily 

 for convenience of description as follows : the skin and its 

 outgrowths, appendages, skeleton, muscular system, nervous 

 system, sense-organs, the food-canal and its outgrowths, 

 the body-cavity, the heart and blood-vessels, the respiratory 

 organs, the excretory system, the reproductive organs. 



Tissues. — To the school of Cuvier we owe the analysis 

 of the animal organism into its component organs ; but as 

 early as 1801 Bichit published his Anatomie Generate, in 

 which the analysis was carried a step farther. He reduced 

 the organs to their component tissues, and maintained that 



