THE CRAYFISH 147 



stomodaeum has grown into a large sac, the proventriculus, 

 in which the cardiac and pyloric divisions can be distinguished. 

 It opens into the mesenteron, now short in length, but pro- 

 duced dorsally into a large saccular lobe lined by hypoblast 

 cells, still distended with yolk. Several digitate diverticula 

 from the sides of the mesenteron indicate the commencement 



IV. 



a/. 



- 35 



Sagittal section through an advanced embryo of Astacus (after Reichenbach). 

 a.abd, superior abdominal artery ; ab.i, inferior abdominal artery ; ao, ophthalmic 

 artery ; An, anus ; Ent, anterior division of the enteron ; Ent 2, dorsal division 

 of the enteron ; ht, heart ; Int, intestine, formed from the proctodaeum Prc ; 

 Liv, outgrowths of the mid-gut which will form the digestive gland ; M y mouth; 

 R, rostrum ; Spg, supraoesophageal ganglion ; St, stomodaeum ; st.a, sternal 

 artery ; XIII, ganglion of the thirteenth segment, or eleventh post-oral 

 ganglion ; XIX, ganglion of the nineteenth segment (seventeenth post-oral 

 ganglion). 



of the digestive gland. The proctodceum is still shut off 

 by a partition from the mesenteron, and is intersegmentally 

 constricted. The vascular system is well developed, and the 

 principal arterial channels are established. The cerebral, 



