Development and Morphology of the Vascular System. 351 



end of the embryonic area into the vascular network on the wall of 

 the yolk-sac. 



2. With the formation of the cephalic and caudal folds each 

 primitive trunk, at first almost straight, is so folded in front and 

 behind that a dorsal and two ventral portions with uniting caudal 

 and cephalic arches are differentiated. 



3. The dorsal part of each trunk is modified in the cephalic region 

 into vessels of the head and neck ; the remainder of each dorsal 

 portion fuses with its fellow of the opposite side to form the greater 

 part of the systemic aorta. 



4. The cephalic or anterior ventral portions are converted into the 

 heart, the ventral part of the arch of the adult aorta, and vessels of 

 the head and neck. 



5. The caudal or posterior ventral portions either fuse together to 

 form a common vitello-allantoic stem, as in rodents, or they remain 

 separate and form the ventral parts of the allantoic arteries as in 

 carnivores, ruminants, and man. 



6. The ventral and dorsal sections at first are united anteriorly by 

 a cephalic arch and posteriorly by a primary caudal arch. Additional 

 cephalic arches are developed subsequently, to be afterwards utilised 

 in the formation of vessels of the head, neck, and upper extremity 

 and of part of the arch of the aorta of the adult, whilst additional 

 arches in the caudal region may also be formed to be utilised as 

 visceral arteries. 



7. The dorsal and ventral extremities of the primary caudal arches 

 remain ; the dorsal extremities take part in the formation of the 

 posterior end of the aorta of the adult; the ventral extremities are 

 utilised in the formation of the ventral portions of the allantoic or 

 hypogastric arteries. 



8. The middle parts of the primary caudal arches disappear and 

 are replaced by " secondary " caudal arches which lie to the outer 

 sides of the Wolffian ducts. In rodents and man the secondary arches 

 are transformed into the common and internal iliac arteries and the 

 dorsal parts of the hypogastric arteries, whilst in carnivores they are 

 probably transformed into the posterior part of the adult aorta and 

 into the internal iliacs and dorsal parts of the hypogastric arteries. 



9. The vessels which are to be looked upon as the posterior con- 

 tinuations of the primitive aorta in the adult in man, rodents, &c, 

 are the common iliac, internal iliac, and hypogastric arteries, and in 

 the carnivores, &c, the internal iliac and hypogastric arteries. 



10. The common and internal iliac arteries are not segmental 

 vessels, their branches may be. 



11. The middle sacral artery is a secondary branch, probably 

 representing fused segmental vessels. 



12. The systemic aorta is formed from the following parts of the 



