TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 495 



Amphioxus, this potential, third germ layer forms a part of the entodermal 

 roof, although the studies of Conklin ('32) have demonstrated that notochord 

 and mesoderm are distinct cellular entities even in the blastula. In contrast 

 to this condition, the notochord and the mesoderm already are segregated as 

 a middle germ layer between the ectoderm and the entoderm in the late 

 vertebrate gastrula. The gastrula of Amphioxus, therefore, has the added 

 problem of segregating the notochordal and mesodermal cells from the ento- 

 derm during tubulation of the major organ-forming areas. 



2. Neuralization and the Closure of the Blastopore 



In the late gastrula of Amphioxus, a longitudinal middorsal plate of cells, 

 the neural plate, elaborated by cell division and extension during gastrulation, 

 represents the future central nervous system (fig. 247E). As the period of 

 gastrulation comes to its end, the blastopore decreases greatly in size (fig. 

 247A-D). The archenteric opening also moves dorsally, coincident with a 

 shifting of the caudal end of the archenteron in such a way that it projects 

 in a dorso-caudal direction (figs. 1 89G, H; 247H). This movement of the 

 archenteron is associated with the migration of the mass of mesodermal cells 

 from the two lateral areas of the blastoporal lips (fig. 247A, B) to the dorso- 

 medial portion of the blastopore (fig. 247C), where the mesoderm comes to 

 lie on either side of the notochord below the neural plate (fig. 247C). As 

 these changes occur, the dorsal area of the gastrula near the blastopore be- 

 comes flattened with a subsequent depression of the neural plate (fig. 247C, 

 D). In sagittal section, the gastrula now appears oval in shape and consid- 

 erably elongated in the antero-posterior direction (fig. 189G, H); in trans- 

 verse view, it is triangular, especially at the caudal end (fig. 247D). 



As the above changes are brought about, the ectoderm of the ventral lip 

 of the blastopore grows dorsad, while that of the lateral lips grows mediad. 

 In this way, the opening of the blastopore is closed by the coming together 

 and fusion of these ectodermal (epidermal) growths (fig. 247D-F). How- 

 ever, the archenteron does not lose its connection, at this time, with the 

 outside environment of the embryo for two reasons: 



( 1 ) As observed above, the caudal end of the archenteron previously had 

 shifted in such a manner that it now projects dorso-caudally; and 



(2) synchronized with the epidermal growth which closes the blastoporal 

 opening (fig. 248A), the neural plate sinks downward, becoming de- 

 tached along its margin from the epidermal area (fig. 248B-D). 



The downward sinking of the neural plate and its detachment from the 

 epidermal layer begins at the dorsal lip of the blastopore and spreads anteriad. 

 (Compare fig. 248D with 248B and C.) Consequently, as the epidermal growth 

 along the lateral lips of the blastopore reaches the area of the sinking neural 



