I 



THE FORE-GUT. 213 



The sternal urea 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, p c). 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 bod}^ 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,/a). 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 



