EARLY DEVELOPMENT OF AMPHIOXUS. 43 



parts diverge in the lateral lips of theaperture\ The portion along the archen- 

 teron is the gastral mesoderm, that around me" blastopore the peristomaLj 



The neural plate becomes depressed along its center and the edges turned^ 

 upward, forming the neural groove. Depression and elevation continue 

 until the two edges meet dorsally in the median plane. Fusion of the 

 edges begins not far from the anterior end and progresses both forward and 

 backward until the entire structure becomes tubular. Thus the neural 

 tube with its central canal is formed (Fig. 22, d}. At the caudal end the cen- 

 tral canal remains in open communication with the archenteron owing to 

 the fact that when the ectoderm grew over the neural plate it also grew over 

 the blastopore. The opening thus left is the neurenteric canal (Fig. 23). 

 So long as the neuropore also persists at the cephalic end of the neural tube 

 there is direct communication between the exterior and the archenteron via 

 the central canal and the neurenteric canal. In Amphioxus the neuropore - 

 persists until the mouth is formed. 



//The depression of the center of the neural pjate produces a depression 

 *also of the notocord rudiment and the mesial edges of the mesoderm bands. 

 One effect of this is an inverted groove, the enteroccel, along each side of the 

 notocord, so that the mesoderm appears to bulge outward (Fig. 22, a, b). 

 The grooves extend almost the entire length of the embryo and speedily 

 grow deeper, the mesoderm intruding between entoderm and ectoderm 

 and becoming clearly differentiated from the notocord and the remainder of 

 the entoderm (Fig. 22, c). Near the cephalic end of the embryo a trans- 

 verse fold drops from the dorsal part of the mesoderm on each side, which 

 closes the groove and delimits the most anterior portion from that imme- 

 diately behind it. The portion thus delimited, with its fellow of the opposite 

 side, constitutes the first pair of mesodermal somites. Another portion is 

 delimited in the same manner to form the second pair of somites. Then 

 the third pair is formed; and so on toward the caudal end of the embryo 

 (Figs. 23 and 24). The development of mesodermal somites therefore takes 

 place from before backward. 



Each somite assumes a cuboidal form and is hollow, the cavity being a 

 portion of the original groove-like enteroccel, and the cells surrounding the 

 cavity comprise a simple cuboidal epithelium. For a short time an opening 

 between the enteroccel and gut cavity remains, but later this is closed as the 

 mesoderm becomes entirely cut off from the entoderm and the latter again 

 forms a continuous lining of the gut. These processes too occur from before 

 backward. 



The fact that the formation of mesodermal somites progresses from before 

 backward, that is, from the cephalic end of the body toward the caudal end, 

 illustrates a fundamental principle of growth. The distinction between gas- 



