762 THE CIRCULATORY SYSTEM 



adult. The arteries to the bilateral appendages arise as modifications 

 of the lateral branches of the intersegmental arteries (fig. 343B, C). 



2) Lateral arteries which are not as truly segmented as are the dorsal 

 intersegmental arteries. They pass laterally into the developing nephro- 

 tomic structures (fig. 343A). The renal and genital arteries of the adult 

 are derived from the lateral series of arteries. 



3 ) Ventral arteries much fewer in number than the above-mentioned series 

 (fig. 343A). The vitelline arteries of the yolk-sac area are the first of 

 these ventral arteries to develop. In the Amniota, the umbilical or 

 allantoic arteries also belong to the ventral series of arteries arising 

 from the dorsal aorta. These vessels pass to the placenta or allantoic 

 areas. The coeliac, superior mesenteric, inferior mesenteric, and um- 

 bilical arteries are the adult derivatives of the ventral series of arteries 

 arising from the primitive dorsal aorta. 



E. Development of the Lymphatic System 



The lymphatic system often is called the white blood circulatory system 

 because red blood cells are not present normally, its blood being composed 

 of a lymph fluid and various types of white blood cells. 



Lymph vessels are present in all gnathostomous vertebrates, particularly 

 in the bony fishes and in amphibia, reptiles, birds, and mammals. They appear 

 to be absent in cyclostomes. The lymphatic system is highly developed in the 

 amphibia where it possesses lymph hearts, which actively propel the lymphatic 

 fluid forward. Lymph hearts are found in the tail region of bird embryos, 

 including the chick. However, lymph flow on the whole is of a sluggish nature. 

 Lymph vessels never join arteries but connect in various regions with the 

 veins. In larval amphibia and in certain adult species of amphibia, these 

 connections with the venous system may be numerous. 



Fig. 342. Modifications of the aortal arches (Continued). (A) Generalized, basic 

 condition of the aortal arches in the chick embryo developed during the first 3'/2 days 

 of incubation. (B) Left lateral view of condition present during latter part of the 

 third day. (C) Schematic representation of changes in aortal arches, dorsal aortae. 

 and the aortal sac of the chick embryo after the first week and a half of incubation. 

 Observe that each external carotid artery arises from the anterior end of a ventral 

 aortic root plus an anastomosis with the common carotid segment. Note further that 

 the right and left sixth aoral arches persist until approximately the tweaty-first day 

 (see diagram D). (Diagram C is based to some extent upon data supplied by Pohlman, 

 1920. Anat. Rec. 18.) (D) Dorsal view of adult condition of aortal-arch and bulbus- 

 cordis derivatives in the developing chick after hatching. (E) Generalized aortal arch 

 condition in mammalian embryo. (F) Dorsal view of aortal arches of about 6 mm. 

 human embryo. (G) Lateral view of same. (This figure redrawn and adapted from 

 Patten, 1946. Human Embryology, Blakiston, Philadelphia, after Congdon.) (H) 

 Dorsal view of aortal arches of 14 mm. embryo. (I) Left lateral view of same. (This 

 figure is redrawn and adapted from Patten, 1946. Human Embryology, Blakiston, Phila- 

 delphia.) (J) Dorsal view of conditions present after birth. (See also Fig. 379.) 



