TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 



501 



entoderm continues slowly in a caudal direction from the anterior end until 

 an embryonic condition is reached approximating about 13 to 14 pairs of 

 mesodermal segments. At this level, the notochord and mesoderm become 

 completely separated from the entoderm. As a result, the enteric or gut tube 

 from this point in its growth posteriad is a separate entity. (See tubulation 

 of mesoderm on p. 505. Anterior to the fourteenth somite, after the notochord 

 and mesoderm separate from the entoderm, the latter grows medially from 

 either side to complete the entodermal roof below the evaginated notochord 

 and mesoderm (fig. 250A). A primitive metenteric tube thus is formed, as 

 shown in figure 249C, whose only opening is that which leads by way of the 

 neurenteric canal (fig. 249A, C) into the neurocoel of the neural tube and 

 from thence to the outside through the anterior neuropore. 



NEURAL PLATE CELLS 



NEURAL PLATE 



EPIDERMAL OVERGROWTH OF NEUR 



PLATE BEGINS AT LATERAL 



BLASTOPORAL LIPS AND GROWS 



ANTERIAD AND MEDIAD CLOSING 



BLASTOPO R E 



FORMING NEURENTERIC CANAL 



Fig. 247. Closure of the blastopore and epidermal overgrowth of neural plate in 

 Amphioxus (original diagrams, based on data supplied by Conklin, '32). (A) Vegetal 

 pole view of early stage of gastrulation, showing general areas occupied by notochordal, 

 entodermal, and mesodermal cells. (B) Same view of gastrula, one hour later, showing 

 triangular form of blastopore. (C) Posterior view of late gastrula. Blastopore is now 

 ovoid in shape and dorsally placed. Gastrula is triangular in transverse section with 

 dorsal surface flattened. (D) Same view, later. Slight epidermal upgrowth, indicated 

 by arrows (a and a') merges with ingrowing epidermal edges along lateral lips of 

 blastopore (b and b') which spreads along epidermal edges of neural plate. (E) Dorsal 

 view a brief period later than (D). Epidermal ingrowth from lateral blastoporal lips is 

 now closing the blastoporal opening, shown in broken lines, and also is proceeding 

 craniad along edges of sinking neural plate. (See fig. 248.) (F, G) Later stages of 

 epidermal overgrowth of neural plate. (H) Sagittal section of (G). 



