CHAPTER XIX. 



THE BODY WALL, EIBS AND STERNUM. 



Bilateral Symmetry of the Body. — Erom a developmental 

 point of view the body is made up of two symmetrical halves; 

 each half of the blastoderm, taking the medullary groove as the 

 line of division, contributes equally to the formation of the body. 

 Each produces a half of the nervous system, each a half of the 

 vascular, muscular and alimentary systems, so that each individual 

 is in reality made up of two identical halves, right and left. 



The Ventral Line of the Body. — The tw.o halves are united 

 along the ventral line from the mouth to the anus (see Fig. 227). 

 In this line are developed the symphysis of the lower jaw, the 

 body of the hyoid bone (copula), the white line of the neck and 

 angle of the thyroid cartilage, the sternum, the supra-umbilical 

 part of the linea alba, umbilicus, infra-umbilical part of the linea 

 alba, symphysis pubis, the septum of the penis, and of the scrotum 

 and perineal raphe. The ventral line is continued forwards on 

 the face between the parts derived from the mesial nasal processes 

 (Chap. I.). 



The idea was at one time prevalent that the whole of this 

 line was formed by the fusion of one somatopleure with the 

 other ; the median ventral line was the suture formed by the 

 union. Such is not the case. The blastoderm, which lies at first 

 like a cap on the yolk-sac (Fig. 75, p. 96), is produced or folded 

 anteriorly to form the fore-gut and the part of the body above the 

 umbilicus ; it is produced posteriorly to form the hind-gut and 

 the part of the body below the umbilicus (Fig. 75). The 

 blastoderm grows out from the umbilicus to form the embryo in 

 much the same way as a soap bubble is blown from the bowl of 



