TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 



497 



plate in the region of the dorsal blastoporal lip, it continues forward along the 

 epidermal margins of the insinking neural plate, growing mesad and fusing in 

 the midline over the neural plate (fig. 247E-G). In this way, the epidermal 

 growth forms a covering for the neural plate. It follows, therefore, that the pos- 

 terior end of the archenteron will now open into the space between the neural 

 plate and its epidermal covering. This new passageway between the epidermal- 

 neural plate cavity and the archenteron is the beginning of the neurenteric 

 canal (figs. 247H; 248A). 



The flattened neural plate, canopied by the epidermal overgrowth, then 

 begins to fold itself into the form of a tube. In doing so, its lateral edges 

 swing gradually toward the middorsal line, as shown in figure 195. The actual 

 grooving and tubulation of the neural plate starts at a point about midway 

 along the embryo at the stage of development shown in figure 247F. It pro- 

 ceeds anteriorly and posteriorly from this point. At its extreme anterior end, 

 the neural tube remains open to the surface as the anterior neuropore (figs. 

 247H; 249A-D). Eventually the caudal end of the neural plate becomes 

 tubulated, and a definite canal is formed, connecting neural and enteric tubes. 

 This canal is the neurenteric canal. The neurenteric canal disappears between 

 the stage of development shown in figure 249C and that shown in figure 249D. 

 The continued caudal growth of the neural tube is accomplished by cell 

 proliferation from the posterior end of the tube and neurenteric canal area. 



MESENCHYME OF HEAD 



NEUR4L GROOVE 



NEURAL GROOVE 



L 



Fig. 243. Sections of pig embryo of about stage shown in fig. 242 (B) and (C). 

 (Modified from Patten: Embryology of the Pig. 3d Ed., Philadelphia, Blakiston, '48.) 

 (A) Line 1, fig. 242C. (B) Line 2, fig. 242C. (C) Line 3, fig. 242C. (D) Line 4, 

 fig. 242C. 



