TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 507 



The lower sclerotomic diverticulum (fig. 25 OD, E) extends up between the 

 myotome and the medially placed notochord and nerve cord, as diagrammed 

 in figure 250F. Its walls differentiate into two parts: 



( 1 ) an inner layer which, together with a similar contribution from the 

 somite on the opposite side, wraps around the notochord and nerve 

 cord and, subsequently, gives origin to a skeletogenous sheath of 

 connective tissue which enswathes these structures; and 



(2) an outer layer which covers the mesial (inner) aspect of the myotome 

 with a fascia or connective tissue covering. 



The outer surface of the myotome does not have a covering of fascia. 



The ventral diverticulum extends between the lateral wall of the splanchno- 

 coel and the epidermal layer of the body wall (fig. 250E, F) and separates 

 the parietal wall of the splanchnocoel from the epidermal wall (fig. 250F). 

 This ventral diverticulum or dermatomic fold, together with the external or 

 parietal wall of the myocoel above, forms the dermatome. The inner and 

 outer layers of the ventral diverticulum gradually fuse to form the cutis or 

 dermal layer of the integument or skin in the ventro-lateral portion of the 

 body, whereas the parietal wall of the myocoel above gives origin to the same 

 dermal layer in the body region lateral to the myotome. The dorsal sclerotomic 

 diverticula form the fin-ray cavities in the dorsal fin. These cavities become 

 entirely isolated from the rest of the myocoelic spaces. Several fin-ray cavities 

 occupy the breadth of a single myotome. The dorsal myotomic portion of the 

 somite thus differentiates into three main structural parts: 



(a) the muscular myotome, 



(b) the mesial sclerotome or skeletogenous tissue, and 



(c) the latero-ventral dermatome or dermal tissue of the skin. 



7. Notochord 



The notochord arises as a middorsal evagination of the primitive archen- 

 teron up to about the stage of about 13 to 14 pairs of somites (fig. 195). 

 Posterior to this region it takes its origin by proliferative growth from a sep- 

 arate mass of notochordal tissue, lying above the gut and between the two 

 mesodermal masses of cells. Its origin posterior to the general area of the 

 thirteenth to fourteenth body segments, therefore, has no relation to the ento- 

 derm. It rapidly develops into a conspicuous skeletal rod, lying below the 

 neural tube and between the mesodermal somites and resting in a slight de- 

 pression along the dorsal aspect of the metenteron or entodermal tubulation 

 (fig. 249E, H). It continues forward in the head region, anterior to the brain 

 portion of the neural tube (fig. 249E). 



(The student is referred to the following references for further details rela- 

 tive to the early development of Amphioxus: Cerfontaine, '06; Conklin, '32; 



