PHYLOGENY OF THE HUMAN HEART. 385 



V. Fifth Period : Vascular System of the Acrania. 



The ventral vessel (intestinal vein) forms, round the developing liver. 

 §ac, the first rudiment of a vena portae system. 



VI. Sixth Period : Vascular System of the Cyclostomi. 



The aingle-chambered heart divides into two chambers ; a posterior 

 ventricle, and an anterior auricle. The lymph-vessel system develops side 

 by side with the blood-vessel system. 



VII. Seventh Period : Vascular System of the Primitive Fishes, or Selachii. 



From the anterior section of the main chamber of the heart arises an 

 artery-stalk or trunk, from which five (?) pairs of arterial arches proceed. 



VIII. Eighth Period : Vascular System of the Mud-fishes. 

 From the last (fifth) pair of arterial arches the lung-arteries develop, 

 as in the Dipneusta. 



IX. Ninth Period : Vascular System of Amphibia. 



The gill-arches gradually disappear with the gills. The right and left 

 aortal arches remain. 



X. Tenth Period : Vascular System of Mammals. 



The separation of the greater from the lesser circulation is complete. 

 The right aortal arch unites with Botalli's duct. 



TABLE £LL 



Ststmiatic Survey of the most important Periods in the Phylogzny 

 op the Human Heart. 



I. First Period : Heart of Chordoma. 



The heart forms a simple spindle-shaped enlargement of the ventral 

 vessel, with an alternating blood-current (as in Ascidia). 



II. Second Period : Heart of Acrania. 



The heart is like that of Chordoma, but the blood-current acquires 

 a constant direction, passing only from back to front. (Retrograded in 

 Amphioxus.) 



