$86 THE EVOLUTION OF MAN. 



III. Third Period : Heart of Cyclostoma. 



The heart divides into two chambers, a posterior auricle (atrium) and 

 an anterior ventricle (yentriculus). 



IV. Fourth Period : Heart of Primitive Fishes. 



From the anterior section of the ventricle is differentiated an arterial 

 stalk (bulbus arteriosus), as in all Selachii. 



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



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

 a right and a left half, as in Dipneusta. 



VI. Sixth Period : Heart of Amvhibia. 



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

 the higher Amphibia. 



VII. Seventh Period : Heart of Protamnia. 



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 J pes. 



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, a3 in Apes and 

 Mao. 



