GASTRULATION PROPER 



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Presumptive 

 neural plate 



Lateral / Lips of blastopore 



Involutional movements during gastrulation outlined on the living gastrula. 



synonymous with elongation (or Strekung of Vogt), and probably 

 occurs in the frog's egg gastrulation. 



Constriction — This term is used to describe the convergence of cell 

 areas resulting in the gradual closure of the blastopore. It is due to a 

 narrowing of the marginal zone and a pull, or tension, of the dorsal lip. 

 This occurs in the frog's egg gastrulation. 



Dorsal convergence — This is the dorsal Raffung of Vogt, used to 

 describe the movement of the marginal zone cell areas toward the 

 dorsal mid-line during involution and invagination. 



With this general introduction to a very fascinating but, as yet, litde 

 understood phase of embryological development, we shall retrace the 

 steps to the late blastula stage and endeavor to anticipate the process 

 of gastrulation in the frog. 



Gastrulation Proper 



To understand the processes to be described as gastrulation, let us 

 list in succession the changes that may be observed in the frog embryo. 



1. Thinning of the gray crescent side of the blastula wall. There 

 appears to be an actual migration of cells from the deeper layers 



