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 



XDC 



Fig. 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 ; /I/, mouth; 

 R, rostrum ; Spg, supraoesophageal ganglion ; .9^, stomodaeum ; st.a, sternal 

 artery ; A'///, 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 proetodseum 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, 



