386 THE EVOLUTION OF MAN. 



III. Third Period : Heart of Cyclostoma. 



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

 an anterior ventricle (ventriculus). 



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 anricle 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 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 



