5o2 embryology: frog 



When it was external it was not distinguishable from the epiblast, 

 but could be shown by staining methods to lie in strips (pre- 

 sumptive areas) across the dorsal surface. As the small cells 



advance, these areas converge upon the 

 lip and turn in over it. The lip is the 

 upper border of the blastopore, which 

 faces backwards : the rest of the 

 border is as yet indefinite and repre- 

 sented by the Hmit of the advancing 

 ectoderm all round the egg. 



All this time the shape of the crescent 

 is changing by its two ends lengthening 

 and curving towards one another till 

 at last they meet to form a circle. By 

 that time the edge of the ectoderm has 

 reached this circle all round its cir- 

 cumference, so that all the yolk is 

 covered except that within a circular area, the definitive 

 blastopore, bordered by a continuous lip and filled by a yolk 

 plug consisting of yolk cells which have not yet been covered. 

 At this stage the embryo, which has hitherto floated with the 



Fig. 491. — The embryo of a 

 frog shortly after the com- 

 pletion of gastrulation, seen 

 from the right side and 

 somewhat from behind. 



blp., Blastopore ; n.f., neural folds. 



n.p. 



n.p. 



,nch. 



nch.. 



end.-- 



'ines. 



..'blL 



end 



mes 



B 



Fig. 492. — Sections of an embryo at about the stage of Fig. 491. 



A, Vertical longitudinal ; B, transverse. 



blc, Blastocoele ; end., yolky cells of endoderm ; ent., gut ; mes., mesoderm ; n.f., neural fold ; n.p., neural 



plate ; nch., notochord ; y.p., yolk plug. 



white pole downwards, begins to rotate so that that pole moves 

 upward through the aspect on which the dorsal hp grew down. 

 (Counter-clockwise in the sections of Fig. 490. This figure shows 

 all the stages in their final relationship ; i.e. as if the rotation 

 had taken place before gastrulation ; the blastocoele in A is in 

 fact upper.) Towards this aspect will be directed the future 



