156 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(4) Vessels; (5) Nerves; (6) Neural segment. Although the 



epiblast and hypoblast are never segmented, yet a definite area 



of each is associated with every body segment. The origin of 

 each element will be taken separately. 



lat dorsi 



ilio-costalis 



vertebra (sclerotome j 



neural canal 



quad. lumb. 



lumb. aponeur. 



Kidney, 

 (intermed. 

 cell-mass) 



nt. ob. 

 somato-pleure 



Fig. 125. 



post. root. gang. 



muscle plate 

 skin plate 

 sclerotome 

 notochord 



—aorta 



( intermed. 



\ cell-mass. 



■splanehno-pleure 



somato-pleure 



coelom 



gut 



Fig. 126. 



Fig. 125. — A transverse section showing the Elements of the 1st Lumbar Segment in 



the Adult. 



Fig. 12(3. — A corresponding section of an Embryo about the end of the 3rd week 



(diagrammatic). 



I. The skeleton of the 11th dorsal segment is represented by 

 the adjacent halves of the 11-12 dorsal vertebrae and the disc 

 between them, for, as already pointed out, the vertebrae are inter- 

 segmental in origin (Fig. 126). The transverse processes, the 

 spinous process and 11th and 12 th ribs are also formed in the 

 septa in front of and behind the 11th segment. The septum in 

 the rectus muscle a little below the umbilicus represents' the 

 inter-segmental septum corresponding to the 11th rib. Sometimes 

 another septum occurs in the rectus, midway between the pubes 

 and umbilicus, marking the lower limit of the 11th segment. 

 The linea alba separates the segments of the two sides. 



In the linea alba or ventral median line of the thoracic region, 

 the sternum is developed. The inter-segmental septa are well 



