106 OUTLINES OF CHORDATE DEVELOPMENT 



tula or early gastrula and any single straight axis of the later 

 gastrula. 



The description of gastrulation from observation of median 

 sagittal sections or hemisections does not give a complete idea of 

 this process. We may return now to a consideration of some of 

 the processes going on in the lateral parts of the gastrula. As 

 the dorsal lip of the blast opore becomes crescentic, the deepen- 

 ing archenteron pushes laterally around through the mass of 

 yolk cells (Fig. 33, B, C, D). But these have already become 

 separated from the ectoderm by the gastrular groove and cleav- 

 age, except only in the region of the lower pole just anterior to 

 the place where the anterior (ventral) lip of the blast opore will 

 form. So that by the time the blastoporal lip becomes circular, 

 i.e., by the time the ventral lip forms, this region has already 

 been divided into ectoderm and endoderm, and therefore the ex- 

 tent of invagination at the ventral lip is greatly limited. The 

 important result of this is that the actually invaginated endo- 

 derm is confined to a broad tongue of cells on the dorsal side 

 (roof of the archenteron) and a ring-like strip extending around 

 within the blastopore lip from the base of this tongue, nar- 

 rowing rapidly toward the ventral side. The archenteron itself 

 is at first a narrow slit, crescentic in cross section, but as it 

 grows up to the animal pole and enlarges, it grows farther lat- 

 erally so as to extend in a wide crescent (in transverse section) 

 about to the level of the equator of the gastrula (Fig. 33, E, F). 

 This leaves the yolk cells as a convex mass projecting into 

 the archenteron from its floor. 



The frog illustrates very well the way in which the process of 

 gastrulation proper is complicated, in the Chordata, by the early 

 formation of certain of the important axial organs which are the 

 chief characteristics of the Chordate group; the formation of the 

 rudiments of these structures is termed notogenesis. Gastrula- 

 tion proper includes only those processes by which the single- 

 layered (monodermic) blastula is converted into the two-layered 

 (didermic) organism, with definitely established ectoderm and 

 endoderm the gastrula. The method by which this is accom- 

 plished may vary in different groups of Chordates; in Amphi- 



