THE BODY WALL, RIBS AND STERNUM. 



283 



a pipe. In an embryo, at the commencement of the 3rd week 

 the greater part of the ventral line is occupied by the umbilicus 

 (Fig. 228). At that time the umbilicus is 3 mm. long, the 



<»*■} 



Symphysis menti- 

 uent. line of neck- 



vent. line of thorax- 



j~\^- 



■stomodaeum 



i / vent, line of I 



J [pharyn. reyionj 



supra-umb. linea alba-* 



umbilicus (j\ 



infra-umb. linea alba — j 



symph. pubis — ^y\r 

 uulu. cleft 1 



nn. raphe -L 



anus — — -Os 



umbilicus 



enital part 

 perineal depression 

 anal part 



\y coccyx 



Fig. 228. 



perm. 



anus — y- 



coccyx 



Fig. 227. 



Fig. 227. — Diagram of the Structures formed in the Median Ventral Line of the Body. 

 Fig. 228. — The Median Ventral Line in an embryo of three weeks, to contrast with 

 the Corresponding Line in the Adult. 



entire ventral line being about 4 mm. At the 6th week the 

 ventral line measures 15 mm., the umbilicus retains its former 

 size, about 3 mm. 



At first the somatopleure shows no trace of segmentation. 

 The paraxial masses of mesoblast become segmented early and 

 form the muscle plates (Fig. 126, p. 156). From each muscle 

 plate of the primitive segments a process grows down into 

 the somatopleure. The somatopleure thus becomes segmented 

 secondarily, the process of segmentation spreading from the 

 dorsal to the ventral side of the plate, but along the median 

 ventral line of the body wall a band of the primitive mesoblastic 



