PHTLOGENT OF THE HUMAN HEART. 385 



V. Fifth Period : Vascular System of the Acraitiim, 



The rentral vessel (intestinal vein) forms, round the deTeloping Krer- 

 MO, the first rudiment of a vena portee system. 



YI. Siafth Period : Vascular System of the Cyclostomi. 



The aingle-ctambered heart divides into two chambers ; ft po8t«rior 

 T©ntricle, and an anterior auricle. The lymph-ressel system derelops side 

 hj side with the blood-vessel system. 



Vn. Seventh Period : Vascular System of the Prim/itive Fishes, or Selachii. 



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

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



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

 From the last (fifth) pair of arterial arches the Inng-exteries develop, 

 as in the Dipneusta. 



IX. Ninth Period : Vascular System of Amphihui. 



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

 aortal arches remain. 



X. Tenth Period ; Vascular System of MammmLs, 



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

 The right aortal arch unites with Botalli's duct. 



TABLE ^LL 



STimfATIC SUBVBT OF THK MOST IMPOKTANT FeEIODS IK THI PhTLOCKWT 



o? THE Human Heart. 



L First Period : Heart of Chordonia, 



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

 Teaeel, with an alternating blood-ourrent (as in Ascidia). 



II. Second Period : Heart of Acrania. 



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

 a constant directicm, petssing only from book to front. (Betrt^raded in 

 Amphioxns.) 



