410 GASTRULATION 



Coincident with the lateral extensions of the original dorsal lip of the blasto- 

 pore to form the lateral lips, a more extensive convergence and involution of 

 presumptive mesoderm located in the lateral portions of the mesodermal cres- 

 cent occurs (fig. 197A, B). The latter mesoderm eventually forms the lateral 

 area of the hypomeric mesoderm of the future embryo (figs. 19 IG; 198B, 

 C). As the lateral lips of the blastopore continue to form in the ventral direc- 

 tion, they eventually reach a point where they turn inward toward the median 

 axis and thus form the ventral lip of the blastopore (fig. 197C). A rounded 

 blastopore, circumscribing the heavily, yolk-laden, entodermal cells, thus is 

 formed. Associated with the formation of the ventro-lateral and ventral blasto- 

 poral lips is the convergence and involution of the ventro-lateral and ventral 

 mesoderm of the gastrula (fig. 191D-F). Accompanying the inward migration 

 of the entoderm in the region of the dorso-lateral lip of the blastopore, there 

 is, presumably, an inward rotation of the entodermal mass which lies toward 

 the ventral blastoporal area. The result of this entodermal movement is the 

 production of a counterclockwise rotation of the entodermal, organ-forming 

 rudiments, as indicated in figure 194B, compared to their relative positions 

 at the beginning of gastrulation, shown in figure 194A. (This counterclockwise 

 rotation is present to a degree also in Amphioxus (fig. 190A-F). In this way, 

 the particular, organ-forming areas of the entoderm become arranged antero- 

 posteriorly in a linear fashion along the embryonic axis. The foregut material 

 now is situated toward the anterior end of the developing embryo, while the 

 stomach, liver, small intestine, and hindgut regions are placed progressively 

 posteriad with the hindgut area near the closing blastopore (fig. 194B). The 

 yolk material lies for the most part within the ventral wall of this primitive 

 archenteron. 



Associated with the axiation of the entodermal rudiments is the axiation 

 of the notochord-mesoderm complex. For example, the anterior segment of 

 the notochord and the pre-chordal plate (i.e., the head organizer) are lo- 

 cated anteriorly in the gastrula, while the more posterior portions of the 

 notochord and adjacent mesoderm (i.e., the trunk organizer) are located in 

 the developing trunk region (fig. 191G). The mesoderm adjacent to the noto- 

 chord eventually will form the somites or primitive mesodermal segments of 

 the embryo (figs. 191G; 217E; 224F). Experimentation, using the Vogt 

 method of staining with vital dyes, has demonstrated that the future, anterior, 

 presumptive somites lie closer to the blastoporal lips in the beginning gastrula, 

 whereas the more posterior, presumptive somites are situated at a greater dis- 

 tance from the blastoporal area. Because of this arrangement, the first or ante- 

 rior pair of presumptive somites moves inward first, the second pair next, etc. 

 The mesoderm of the future somites in this way is arranged along the notochord 

 in an orderly sequence from the anterior to the posterior regions of the gas- 

 trula (fig. 169, somites 1, 2, 3, 4, etc.). Consequently, axiation and extension 

 of the somitic mesoderm occur along with the antero-posterior arrangement of 



