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a small median groove opening into the canal is found between them. 
Cross sections through this region now resemble those made at an 
earlier stage through the region where the dorsal wall of the alimen- 
tary tanal was continued laterally into the anterior wall of the mandi- 
bular cavities. Here as there, the thickened wall of the canal bounds 
a dorsal groove, and is continuous with the rudimentary mesoderm. 
Fig. 4 is a sagittal section passing, at this stage, through the 
anterior end of the alimentary canal near its axial line, and showing, 
between the points a’ and 7’, the relations above described. At d the 
depression in the floor of the neural plate is seen. At a’ the anterior 
end of the alimentary canal is continued dorsally in a groove, g, which 
at 7’ passes out of section, owing to a slight curve in the long axis 
of the embryo. 
Fig. 4. Fig. 5. 
Fig. 4. Sagittal section showing the anterior extremity of the chorda canal. a me- 
dian Anlage of the „anterior‘‘ head cavities. 1° median Anlage of the praemandibular 
head cavities. g chorda groove. d depression in the floor of the neural plate. n. pl- 
neural plate. mes mesoderm. aiz wall of alimentary canal. ect ectoderm. 
Fig. 5. Sagittal section showing the reduction of the anterior extremity of the ali- 
mentary canal. dr brain. ect ectoderm. n. el closure of neural plate. a’ median An - 
lage of the anterior head cavities. inf infundibulum. Ch chorda. ali alimentary canal. 
m 1 median portion of praemandibular cavities. m 2 median part of mandibular cavities. 
The lateral edges of the neural plate now begin to curve upward, 
and ultimately meet and fuse. The anterior limit of this fusion, which 
is just anterior to the extremity of the primitive alimentary canal 
(fig. 5 n. cl.), is the Anlage of the optic stalk. The depression in 
the floor of the neural plate is now seen to be the beginning of the 
cranial flexure, and its fundus forms the rudiment of the infundibulum, 
Fig. 5 shows the condition of the alimentary canal at this stage. 
A portion of its ventral wall is still seen at a’ anterior to the Anlage 
