506 DEVELOPMENT OF PRIMITIVE BODY FORM 



the mesoderm on either side of the embryo produces a condition wherein the 

 somites of either side may be slightly intersegmental in relation to the somites 

 on the other side (fig. 249H). 



During its later development, each somite grows ventrally (fig. 250B). 

 That portion of the somite contiguous to the notochord and neural tube 

 thickens and forms the myotome. The region of the somite near the epidermal 

 ectoderm is called the somatic or parietal mesoderm, while that associated 

 with the entoderm forms the visceral or splanchnic mesoderm (fig. 250C). 



As the myotome enlarges, the coelomic space becomes more and more 

 displaced ventrally, and most of it comes to lie on either side of the enteron 

 (metenteron). (See fig. 250D.) This ventral coelomic space forms the 

 splanchnocoel, while the dorsal space, lateral to the myotome, is known as 

 the myocoel. Eventually, the splanchnocoels of each pair of somites push 

 ventrally to the lower portion of the enteron, where they ultimately fuse 

 (fig. 250D-F). Gradually the splanchnocoels of each segment fuse antero- 

 posteriorly and in this way a continuous, antero-posterior, splanchnocoelic 

 space below and around the gut tube is formed. Tubulation or the formation 

 of a continuous, antero-posterior, coelomic cavity thus is effected by fusion 

 of the splanchnocoels of the respective somites on either side (fig. 250F). A 

 horizontal septum, the intercoelomic membrane also appears, separating the 

 myocoels above from the splanchnocoelic cavity below (fig. 250D). 



6. Later Differentiation of the Myotomic (Dorsal) Area 



OF the Somite 



While the above events are taking place in the ventral portion of the somite, 

 the upper, myotomic region undergoes profound modification. 



As shown in figure 250D, the myotomic portion of the somite has two un- 

 equally developed areas: 



( 1 ) a medial muscular portion, the myotome and 



(2) the laterally placed, thin-walled, parietal part which surrounds the 

 coelomic space, or myocoel. 



The muscular portion enlarges rapidly and, as seen in figure 250E and F, 

 forms the muscle plate or myotome of the adult. These muscle plates very 

 early assume the typical > shape characteristic of the adult. On the other 

 hand, the myocoelic portion contributes important connective or skeletal 

 tissue to the framework of the body. In each segment, the wall of the myocoel 

 gives origin to three diverticula as follows: 



(a) a lower sclerotomic diverticulum, 



(b) a ventral diverticulum, and 



(c) a dorsal sclerotomic diverticulum. 



