EMBR YONIC DE VEL 0PM EN T. 



127 



ra.d-, 



,-l.ad 



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 slight 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 eventually lose 

 all connexion with the ar- 

 chenteron (Fig. 71). Theali- 

 mentary canal now no longer 

 reaches to the anterior ex- 

 tremity of the body. Very 

 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 Fi g. 70 . - Embryo, with nine pairs of 



the left, and the epithelium primitive somites seen in optical section 



from the ventral surface, to show the 

 lining its walls is losing its origin of the head-cavites. (After HAT- 



C(~"l-TT?TC \ 



original cubical character, rM ' d . Right head-cavity. i.a.d. Left 



the inner ends of the Cells head-cavity, my.c :. Myocoelomic pouches 



(pnmtive somites), arc. Archenteron. 



are rounding off, and in fact 



it is being converted from a cubical to a flattened pavement 

 epithelium (Figs. 63 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 



