DESCENT WITH CHANGE 



365 



next higher group, the Amphibia (Frogs, Toads, etc.), the auricle 

 is divided into two parts, while the ventricle remains as before. 

 Thus these forms have a three-chambered heart. Passing to the 

 Reptiles, we find that most of the Lizards, Snakes, and Turtles 

 have the ventricle partially divided into two chambers, while 

 the more specialized Crocodiles and Alligators have a complete 

 partition and therefore a four-chambered heart. This is the con- 



A B ** 



Fig. 235. — Embryos in corresponding stages of development. A, Fish 

 (Shark); B, Bird; C, Man. g, gill slits. (From Scott.) 



dition in all adult Birds and Mammals, but the significant fact 

 is that, in the development of the heart of the individual Bird and 

 Mammal, embryonic stages succeed each other which parallel in 

 a general though remarkable way this sequence from a two- 

 chambered to a four-chambered condition as exhibited in the 

 adults of the lower Vertebrates. (Figs. 120-122.) 



Or take the development of the brain in the Vertebrate series. 

 Even in the human embryo the fundament of the brain arises by 

 simple transformations of the anterior end of the neural tube, 



