TUBULATION OF ORGAN-FORMING AREAS 



485 



vagination from the epidermal (ectodermal) tube (fig. 242G). Similarly, the 

 caudal region of the hindgut rudiment contacts the proctodaeal invagination 

 of the epidermal tube, while a tail gut extension continues into the tail (fig. 

 217). 



The formation of definitive walls of the midgut area in embryos developing 

 from the flattened gastrular condition (including the higher mammals which 

 do not possess large amounts of yolk substance) occurs as follows: 



(1) Where the entoderm of the midgut terminates on either side of the 

 notochord at the end of gastrulation, it grows mesad from either side 



»irv'fir:si5S:;:-.-. 





Fig. 235. Chick embryo of 9 to 10 pairs of somites. (Approximating Hamburger and 

 Hamilton, '51, stage 10; 33 to 38 hours of incubation.) (A) Surface view, unstained. 

 (B) Stained preparation. (C) Median sagittal section. Observe the following: heart 

 is bent slightly to the right; three primary brain vesicles are indicated; foregut touches 

 infundibular outgrowth of prosencephalon; first indication of downward bending of the 

 head outgrowth, i.e., the cephalic (cranial) flexure is evident. (D) Same, showing 

 major organ-forming areas. 



