THE FOEMATION. OP THE LAYERS IX AMPHIOXUS. 301 
more posterior and dorsal pouches give rise to all the other 
somites. The first pair of somites therefore stand 
in a different category to all the rest. Tliis is true 
not only of their origin as independent evagina- 
tions of the archenteric wall, but as we shall 
presently see of all their subsequent history. I 
compared these somites in 1898 to the collar cavities of 
Balanoglossus, and all my subsequent research has con- 
firmed me in this view. The more posterior pair of pouches 
will then be equivalent to the trunk cavities of Balano- 
glossus, and the varying diameter which these show as we 
folloAv them backwards through this series is the first indica- 
tion of their approaching division into the posterior somites, 
which is the great point in which Amphioxus exhibits an 
advance on the condition represented by Balan oglossu s. In 
the stage under consideration there seems to be a constriction 
of the trunk cavity in one place, b u t throughout its whole 
extent it is freely open to the gut. This constriction 
is an indication of the constriction of the first of the posterior 
somites from the front end of the trunk cavity. In fig. 11 c 
we observe in the mid-dorsal line of the archenteron the first 
indication of the formation of the notochord (c/i.). The arch- 
enteric wall is here arched slightly upwards, and the yolk- 
granules are being rapidly reduced in number so that the cells 
have become relatively clear. Here we have the explanation 
of that difference between the roof and the fioor of the archen- 
teron as seen in median sagittal sections, on Avhich Lwoff and 
Cerfontaine founded such top-heavy conclusions. Fig. lid 
is taken from near the hind end of the embryo. The nerve- 
plate is completely covered in by the union of the ectodermic 
folds above it, so that we now have a neural canal (n. c.). 
The archenteron shows a division into two storeys, exactly as 
Legros has described for his abnormal embryos, but which 
is here described for the first time in a normal embryo 
at least of this age. The lower division (n) is the anal 
diverticulum, which at a much later stage will open to the 
exterior. The upper division (fd) leads to the still open 
