408 GASTRULATION 



somewhat older embryo becomes, in some cases, normally incorporated in 

 the neural tube of the host. Similar interchanges of cells of the late blastula 

 have demonstrated that almost any part, other than the presumptive entoderm, 

 can be interchanged without disturbing the normal sequence of events. How- 

 ever, as gastrulation progresses, interchange from epidermal to neural areas 

 continues to be possible during the early phases of gastrulation (fig. 196C, D) 

 but not at the end of gastrulation. Similar changes occur also in the mesodermal 

 area. Pronounced physiological changes thus occur in the presumptive organ- 

 forming aicas of the entire epiblastic region during gastrulation. 



b. Gastrulation 



1) Emboly. As gastrulation begins, a small, cleft-like invagination appears 

 in the entodermal material of the presumptive foregut area. This invagination 

 is an active inpushing of entodermal cells which fold inward and forward 

 toward the future cephalic end of the embryo (fig. 191B-E). The upper or 

 dorsal edge of the cleft-like depression visible at the external surface forms 

 the dorsal lip of the blastopore (fig. 19 IB). In this connection, study dia- 

 grams in figure 197. The pre-chordal plate cells are associated with the form- 

 ing dorsal roof of the archenteron and, therefore, form a part of the invagi- 

 nated material shortly after this process is initiated. 



As the entodermal material migrates inward and the initial dorsal lip is 

 formed, notochordal cells move posteriad to the dorsal lip and involute to 

 the inside in close association with the pre-chordal plate cells. Also, the more 

 laterally situated, notochordal material converges toward the dorsal lip and 

 gradually passes to the inside, as gastrulation progresses, where it lies in the 

 mid-dorsal region of the embryo. (See arrows, figs. 188A; 19 IC, D). Here it 

 begins to elongate antero-posteriorly (i.e., it becomes extended) and forms a 

 narrow band of cells below the forming neural plate (fig. 191C-G). 



With the continuance of gastrulation, the entodermal material moves more 

 extensively inward (cf. fig. 191C-E) and the entodermal mass of yolk-laden 

 cells below the site of invagination begins to sink or rotate inwardly. The 

 dorsal blastoporal lip, therefore, widens considerably (fig. 197 A, B). In many 

 Amphibia the inner surface of the entoderm, as it progresses inward, forms 

 a cup-like structure which actually engulfs the blastocoelic fluid (fig. 191 B-D ) . 

 It is not clear whether this cup-like form is produced by active inward mi- 

 gration of entodermal cells or whether it may be due in part, at least, to 

 constrictive forces at the blastoporal lip. 



Synchronized with the events described above, the presumptive somitic 

 mesoderm, located externally along either side of the notochordal area of the 

 early gastrula, migrates (converges) toward the forming dorso-lateral lips of 

 the blastopore (fig. 197A, B, broken arrows). Upon reaching the blastoporal 

 edge, the mesoderm moves over the lip (involutes) to the inside. However, 

 the mesoderm does not flow over the lip to the inside as a part of the entoderm 



