THE FORE-GUT. 213 
The sternal area of the embryo gradually enlarges 
until it occupies one hemisphere of the yelk; in other 
words, the thickening of the epiblast gradually extends 
outwards. Just in front of the blastopore, as it closes, 
the middle of the epiblast grows out into a rounded 
elevation (fig. 58, ta; fig. 59, ab), which rapidly increases 
in length, and at the same time turns forwards. This 
is the rudiment of the whole abdomen of the crayfish. 
Further forwards, two broad and elongated, but flatter 
thickenings appear; one on each side of the middle line 
(fig. 58, pc). As the free end of the abdominal papilla 
now marks the hinder extremity of the embryo, so do 
_, these two elevations, which are termed the procephalic 
lobes, define its anterior termination. The whole sternal 
region of the body will be produced by the elongation of 
that part of the embryo which lies between these two 
limits. 
A narrow longitudinal groove-like depression appears 
on the surface of the epiblast, in the middle line, between 
the procephalic lobes and the base of the abdominal 
papilla (fig. 58, C—F, mg). About its centre, this groove 
becomes further depressed by the ingrowth of the epi- 
blast, which constitutes its floor, and gives rise to a 
short tubular sac, which is the rudiment of the whole fore- 
gut (fig. 57, C, and fig. 58, E, fg). At first, this epiblastic 
ingrowth does not communicate with the archenteron, but, 
after a while, its blind end combines with the front and 
lower part of the hypoblast, and an opening is formed by 
