158 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



membranes and skin. The segmental artery joins at its termina- 

 tion with a ventral longitudinal vessel, the deep epigastric. The 

 primitive arrangement in vertebrates appears to have been a 



"V/ dorsal br. 



l ^sp br./ 

 \^ij>^MercosL artery 



r - ~^-aorta 



^it-gut 



ventral anastom. 



Fig. 127. — The distribution of a typical Segmental Artery. 



•dorsal and ventral longitudinal vessel, with the segmental artery 

 passing between them. The vertebral, ascending cervical, deep 

 ■cervical, ascending lumbar and lateral sacral arteries are examples 

 of the anastomoses that may arise between segmental arteries. 



Segmental arteries also arise from the aorta to supply the 

 structures formed from the intermediate cell mass (the kidney, 

 testis, ovary, etc., Figs. 125 and 127). As a rule only one renal 

 segmental artery persists, but frequently accessory renals are seen. 

 These are persistent embryonic vessels of the several segments 

 from which the mesoblast of the kidney arose. The splanchno- 

 pleure shows no trace of segmentation ; hence its vessels (coeliac 

 axis and mesenteric) are not segmental in origin. 



IV. The Nerve Elements of the 11th Segment (Fig. 128). — 

 (1) Although the spinal cord of the human embryo shows no 

 ■certain sign during development of being definitely divided into 

 segments corresponding to those of the body, yet from whatwe know 

 ■of its condition in embryoes of other animals and from clinical 



