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anterior margin of the neural plate. At this stage, the first few pairs 
of mesodermic somites have been formed, and the mesoderm extends 
forwards from these somites in a pair of lateral plates which fuse with 
the endoderm in the dorsal wall of the alimentary canal. Somewhat 
later, anterior cavities appear in the mesodermic plates, which are 
then seen to end with the mandibular cavities, each of which is 
partially divided into an anterior and a posterior portion by a septum 
extending laterally from the median wall of the cavity (fig. 3 sept). 
Sections lateral to that given in fig. 3 show that the cavities marked 
2 and 2 unite with one another, and consequently that the septum 
is incomplete. Posteriorly the walls of the mandibular cavities are 
quite free from cornection with the alimentary canal, at the stage 
represented in fig. 3, while anteriorly the median dorsal wall of the 
cavities is still directly continued into the dorsal wall of the alimen- 
tary canal. 
MEZ 
SZ 
Fig. 3. Section through an embryo in which the mandibular cavity is forming. 
2 and 2’ mandibular cavities. 3 third head cavity (vAN WıIJHE). 9, 10 head cavities. 
n. pl. neural plate. mes. so line of mesodermie somites. ali alimentary canal. sept sep- 
tum in mandibular cavity. 
In the region of the posterior somites, the notochord has been 
extruded from the endoderm, but as it passes forwards, it gradually 
sinks into the dorsal wall of the alimentary canal, and ends anteriorly 
in the thickened mass of tissue which here forms the median connection 
between the walls of the mandibular cavities. Beyond the anterior 
limit of the mesoderm, the alimentary canal originally extends forwards 
as a simple tube showing no further differentiation than a slight 
thickening in its dorsal wall continuous with that in which the chorda 
ends. Soon, however, the floor of the neural plate at the anterior 
end of the embryo, begins to sink in its median portion. This takes 
place before the sides of the plate curve upward anticipating the 
formation of a closed tube. Corresponding to the depression in the 
floor of the neural plate, the cells of the alimentary canal below, are 
closely crowded upon one another, while the lumen they enclose gra- 
dually disappears. Meanwhile those cells which here formed the 
thickened mid-dorsal wall of the canal have ®xtended laterally, and 
