ESTABLISHMENT OF THE ENTODERM 



25 



The changes in the blastula which indicate the approach of 

 gastrulation are, first, a thinning of the blastoderm at its caudal 

 margin and, second, freeing of the blastoderm-^from the yolk 



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in the same region (Fig. J, D). The separation of the blasto- 

 derm from the yolk is evidenced in surface views by a crescentic • 

 gap in the posterior quadrant of the zone of junction (Fig. 7, A). 



