386 THE EVOLUTION OF MAN. 



III. Third Period : Heart of Cyclostoma. 

 The heart diTicles into two chambei-s, a posterior auricle {atrium) and 

 nn anterior ventricle (yentriculus). 



lY. Fourth Period : Heart of Primitive Fishes. 

 From the anterior section of the rentricle is differentiated an arterial 

 stalk (bidius arteriosus), as in all Selachii. 



V. Fifth Period : Heart of the Mud-jishes. 



The anricle divides, by an imperfect and interrupted partition, into 

 a right and a left halt, as in Dipneusta. 



VI. Sixth Period : Heart of Amvhihia. 



The partition between the right and left auricles becomes complete, as in 

 the higher Amphibia. 



VII. Seventh Period : Heart of Protartmia. 



The main chamber of the heart divides, by an incomplete partition, into 

 a right and a left half, as in Reptiles. 



VIII. Eighth Period ; Heart of Monotrema. 



The partition between the right and left ventricles becomes complete, as 

 in all Mammals. 



IX. Ninth Period : Heart of Marsupials. 

 The valves between the auricles and ventricles (atrio-ventricular valves), 

 together with the connecting filaments and papillary muscles belonging to 

 them, are differentiated from the muscular masses of Monotremes. 



X. Tenth Period : Heart of Jpes. 

 The main axis of the heart, lying in the central line of the body 

 becomes oblique, so that the apex is turned to the left, as in Apes and 

 Man. 



