EMBRYOLOGY OF TADPOLE AND CHICK 



567 



Just in front of the neurenteric canal there is an enlargement also, 

 which forms the hind-gut or rectal portion of the intestine. 



The mid-gut is, as in the chick embryo, that small portion in direct 

 connection with the yolk. 



It will be remembered that the true mouth forms in chordates as 

 a secondary inpocketing of ectoderm. In the frog, the outpocketing 

 from the fore-gut which is to meet the ectodermal inpocketing is seen 

 just below the fore-brain (Fig. 329, pharyngeal membrane). This is the 

 region where the mouth will form later. 



The liver will be seen as a ventral outgrowth beneath the anterior 

 end of the yolk mass (Fig. 328). 



In sections, the rudiments of the first two or three visceral pouches 

 can be seen as vertical outgrowths from the sides of the pharyngeal 

 walls (Figs. 295 and 330). The pouches extend to the ectoderm with 



B. 



Fig. 330. 



A. Horizontal section through an embryo frog some time before hatch- 

 ing, showing the optic vesicles springing from the sides of the fore-brain the 

 three anterior pairs of gill-slits, and five pairs of mesoblastic somites. B A 

 similar section through a tadpole shortly after hatching. The head is cut in 

 a lower plane than in A, so only a small part of the anterior end of the brain 

 appears in the section, a 1 , the mandibular arch ; o a , the hyoid arch ; a 3 , the 

 first branchial arch ; bv, blood-vessel in first and second branchial arch ; eg 

 external gills ; ent, enteron ; fbr, fore-brain; mch, branching mesenchyme 

 cells ; na, nasal pits ; nch, notochord ; nps, peritoneal funnel ; op, optic vesicle ; 

 ph, pharynx ; pnp, pronephros ; som, mesoblastic somites which in B are 

 converted into muscle. I, mandibulo-hyoid slit; //, hyo-branchial slit; III-V 

 branchial slits. (After Bourne.) 



