EM BR 1 ■a^ 'IC DE I EL 0PM EXT. 



127 



r.a.d-^ 



The archenteron extends at first to the front end of the 

 body. Its anterior portion, after the formation of several 

 mesoblastic somites, becomes marked off from the hinder 

 region by a shght constriction, which gradually becomes 

 deeper and deeper (Fig. 70), until finally the whole of this 

 portion of the archenteron is divided into two separate 

 sacs, which eventuall)' lose 

 all connexion with the ar- 

 chenteron (Fig. 71 ). The ali- 

 mentary canal now no longer 

 reaches to the anterior ex- 

 tremity of the body. 'S/ery 

 soon after their separation 

 from the archenteron these 

 sacs enter upon a series of 

 changes by which their origi- 

 nally symmetrical disposi- 

 tion is entirely destroyed. 



Already in Fig. 71 it can 

 be noticed that the right 

 sac is becoming larger than pig. ^o.-Embn^o, with nine pa.rs of 



the left, and the epithelium Prim't'^'e somites, seen in optical section 

 ^ from the ventral surface, to show the 



lad 



y?y.c 



—.-'m—arc 



lining its walls is losing its 

 original cubical character, 



origin of the head-cavites. (After H.\T- 

 SCHEK.) 



r.a.d. Right head-cavity. l.a,d. Left 

 the inner ends of the cells liead-cavity. m;'.^. Myoccelomic pouches 



(primtive somites), arc. Archenteron. 



are rounding off, and in fact 



it is being converted from a cubical to a flattened pavement 

 epithelium (^Figs. 6^ C and 64). The left sac, on the con- 

 trary, retains its original form and dimensions for a long- 

 time. During the asymmetrical changes affecting the two 

 sacs, which take place coincidently with the formation of 

 the snout, the left one comes to lie transversely below the 

 notochord, while the right sac becomes greatly enlarged 



