ectoderm overlapping 



neural plate 



mesoderm 



neural plate folding 

 notochord 

 somite ridge 

 enterocoel 



-gut 



neural tube. 



EARLY NEURULA 



B 



NEURULA 



C LATE NEURULA 



neuropore 

 anterior diverticula 

 notochord 



anterior process first somite 



neural canal 



new somites forming from mesoderm ridge 



neurenteric canal 

 mesoderm strand 



archenteron with evaginating somites 



U LATE EMBRYO 



Figure 7-3. Development of neural tube and mesoderm in Amphioxus. (After Nelsen, 1 953, and 

 Huettner, 1949) 



Anterior to the first coelomic pouches, bilateral dorsal 

 diverticula appear (at a stage of about six pairs of pouches) 

 and extend upward next to the notochord (Figure 7-3 D). 

 The left one, located on the median line in front of the gut, 

 forms the preoral pit opening to the surface of the body on 

 the left side. The right one lies above it but below the noto- 

 chord and forms the rostral coelom. 



As the animal develops, it becomes pointed anteriorly, 

 expanded in the pharyngeal region, and tapers to a thin 

 posterior end with a small caudal fin. An oval mouth opens 

 through the left body wall into the anterior end of the 

 pharyngeal cavity (Figure 7-4). The first pharyngeal open- 

 ing, or fenestra, appears ventrally and to the right, and is 

 the most anterior of what is to be the series of the left side. 

 Other fenestrae arise behind, and these tend to become more 

 ventral in position. Fourteen openings appear before the 

 right-side series begins to form. With the appearance of eight 

 openings on the right side, the fenestrae shift around toward 

 their final position. The first and the last five of the left-hand 

 series close up, equalizing the numbers of the two sides. 



While the pharyngeal fenestrae are appearing, the endo- 

 style (Figures 1-4 A,B and 7-4) begins to form in the roof 

 of the pharynx. At first, it is a nearly transverse bilobed 

 thickening, then it is V-shaped with the apex posterior, and 

 finally an anteroposterior band. This structure shifts with 

 the pharyngeal fenestrae at metamorphosis to its midventral 

 position. 



In the right wall of the pharynx, anterior to the first fenes- 

 tra, a vertical groove invaginates to form a tube which dor- 

 sally remains open to the pharynx but ventrally closes and 

 opens to the exterior near the anteroventral margin of the 

 mouth. This organ, the club-shaped organ, is of unknoviTi 

 function and disappears with the appearance of the right 

 row of fenestrae; it has been equated with a pair of slits. 



At about this stage, metapleural folds appear (Figure 1-4), 

 with that of the left side before the right. These folds, grow- 



interncl opening club-shoped organ 

 Hatschek's nephridium \ notochord 



neural tube 



Kollicker's pit 



Hatschek's fossa 

 endostyle (on right wall) 



club-shaped organ 

 exterior opening 



pharyngeal openings 



Figure 7-4. Anterior end of larva with 1 1 (left) pharyngeal openings. 

 (After Goodrich) 



196 



EMBRYOGENESIS OF THE CHOROATES 



