402 GASTRULATION 



E. Chemodifferentiation and the Gastrulative Process 



In the previous chapter it was observed that certain areas of the amphibian 

 blastula are foreordained to give origin to certain organ rudiments in the future 

 embryo because of their position and not because of their innate physiological 

 condition. This condition is true of the future neural plate ectoderm and epi- 

 dermal ectoderm. During the conversion of the late blastula into the late gas- 

 trula, these areas become changed physiologically, and they no longer are 

 determined in a presumptive sense but have undergone changes which make 

 them self-differentiating. This change from a presumptively determined con- 

 dition to a self-differentiating, fixed state is called determination and the 

 biochemical change which effects this alteration is known as chemodifferen- 

 tiation (see Chap. 8). 



Chemodifferentiation is an important phenomenon during gastrulation. As 

 a result of the physiological changes involved in chemodifferentiation, re- 

 strictive changes in potency are imposed upon many localized cellular areas 

 within the major, organ-forming areas. In consequence, various future organs 

 and parts of organs have their respective fates rigidly, and irrevocably deter- 

 mined at the end of gastrulation. The gastrula thus becomes a loose mosaic 

 of specific, organ-forming areas (figs. 194, 205). Consequently, the areas of 

 the beginning gastrula which possess competence (Chap. 8) become more and 

 more restricted as gastrulation proceeds. ChemodifTerentiation apparently oc- 

 curs largely through inductive (evocative) action. 



F. Gastrulation in Various Chordata 



1. Amphioxus 

 a. Orientation 



Consult figures 167, 189, and 190 and become familiar with the animal- 

 vegetal pole axis of the egg, the presumptive organ-forming areas, etc. 



b. Gastrulative Movements 



1) Emboly. As gastrulation begins, a marked increase in mitotic activity 

 occurs in the cells of the dorsal crescent, composed of presumptive noto- 

 chordal and neural plate cells, and also in the cells of the ventral crescent 

 or future mesodermal tissue. The general ectodermal cells or future epidermis 

 also are active (figs. 167, 189, 190B). The entodermal cells, however, are 

 quiescent (Conklin, '32). Accompanying this mitotic activity, the entodermal 

 plate gradually invaginates or folds inwardly into the blastocoel (figs. 189, 

 190). In doing so, the upper portion of the entodermal plate moves inward 

 more rapidly and pushes forward toward a point approximately halfway be- 

 tween the polar body (i.e., the original midanimal pole of the egg) and the 

 point which marks the anterior end of the future embryo (observe pointed 

 end of arrow, fig. 189). Shortly after the inward movement of the entodermal 



