PHYLOGENY OF THE HUMAN HEART. 385 



V. Fifth Period : Vascular System of tlie Acrania. 



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

 sac, the first rudiment of a vena portae system. 



VI. Sixth Period : Vascular System of the Cyclostomi, 



The single-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-fistes. 

 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-archea 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 XLI. 



SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYIOGEXV 

 or THE HUMAN HEART. 



I. First Period : Heart of Chordonia. 



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 Chordonia, but the blood-current acquires 

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

 Ampliioxus.) 



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