446 GASTRULATION 



culiar, particularly the behavior of the entoderm. But little study has been 

 devoted to the group; as a result, our knowledge is most fragmentary. Elusive 

 and burrowing in their habits and restricted to a tropical climature, they do 

 not present readily available material for study. Brauer, 1897, described blastu- 

 lation and gastrulation in Hypogeophis alternans. Our information derives 

 mainly from this source. 



In some respects gastrulation in Hypogeophis is similar to that in teleost 

 and bony ganoid fishes, while other features resemble certain cellular activities 

 in other Amphibia and possibly also in higher vertebrates. For example, the 

 blastoderm behaves much like the flat blastoderm of teleost fishes, for a dorsal 

 blastoporal lip or embryonic portion of the germ ring is formed toward which 

 the notochordal and mesodermal materials presumably migrate, involute, and 

 thus pass to the inside below the epiblast layer (fig. 214A, B). Also, the rapid 

 epiboly of the presumptive epidermal area around the yolk material (or yolk 

 cells) is similar to that of teleost fishes and of the bony ganoid, Amia (fig. 

 214C-E). However, the behavior of the entodermal cells differs markedly 

 from that of teleosts. In the first place, there is a double delamination whereby 

 the solid blastula is converted into a condition having a blastocoel and a gas- 

 trocoel (fig. 214A). These processes occur concurrently with the gastrula- 

 tive phenomena. Blastocoelic formation resembles somewhat the delaminative 

 behavior of the entoderm in reptiles, birds, and mammals, for the entodermal 

 layer separates from the deeper areas of the epiblast layer. The formation of 

 the gastrocoel (archenteron) is a complex affair and is effected by a process 

 of hollowing or space formation within the entodermal cell mass as indicated 

 in figure 214A. The arrangement of the entodermal cells during later gastrula- 

 tive stages resembles the archenteron in the late gastrula of other Amphibia. 

 The archenteron possesses a heavily yolked floor, with the roof of the foregut 

 region complete, but that of the archenteron more posteriorly is incomplete, 

 exposing the notochord to the archenteric space (fig. 214A-C). 



G. The Late Gastrula as a Mosaic of Specific, Organ-forming 



Territories 



It was observed above that the presumptive organ-forming areas of the late 

 blastula become distributed in an organized way along the notochordal axis 

 during gastrulation. Further, while an interchangeability of different parts of 

 the epiblast of the late blastula is possible without upsetting normal develop- 

 ment, such exchanges are not possible in the late gastrula. For during gastru- 

 lation, particular areas of the epiblast become individuated by activities or 

 influences involved with induction or evocation. (The word "evocation" was 

 introduced by Waddington and it has come to mean: "That part of the mor- 

 phogenetic effect of an organizer which can be referred back to the action of 

 a single chemical substance, the evocator." See Needham, '42, p. 42.) As a 



