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TEXT-BOOK OF EMBRYOLOGY. 



is observed. In the completed blastula it was noted that the yolk pole 

 was directed downward owing to the slightly higher specific gravity of the 

 yolk. During gastrulation it is obvious that the center of gravity of the 

 whole mass is shifted. This can readily be seen if one follows the changes in 

 sagittal sections (Fig. 31). As a result the whole structure rotates through 

 an angle of about 90 degrees (Fig. 35). The blastopore therefore assumes 

 a position which is nearly on a level with the center of the gastrula. After 



FIG. 35. Diagrams of median sagittal sections through an eight-cell stage and four stages during 

 gastrulation of the frog's egg. Kopsch, from Kellicott. The arrow marks the vertical. 

 If one compares the shaded parts of the figure with the fixed vertical line it is seen that the 

 gastrula rotates through an angle of about 90 degrees in the counter-clockwise direction. 



the rotation the dorsal and ventral sides and the cephalic and caudal sides 

 (ends) of the gastrula and of the future embryo are fixed. 



The completed gastrula is still spherical, but then at once it begins to 

 elongate in the direction of the axis drawn from the blastopore to the opposite 

 pole. The dorso-caudal region is drawn out into a bud-like structure; the 

 dorsal side becomes flat or slightly concave in the cephalo-caudal direction; 

 the ventral side remains broadly convex owing to the presence of the yolk in 



