CHAPTER SEVEN 

 Gastrulation 



Significance of the Germ Layers Pre-gastrulation Stages 



Origin of Fate Maps Definition of the Major Processes of 



Gastrulation as a Critical Stage in Gastrulation 



Development Gastrulation Proper 



Gastrulation is that dynamic process in early development which 

 invariably results in the transformation of a single-layered blastula 

 stage to a didermic or 2-layered embryo. It involves, but is inde- 

 pendent of, mitosis. The process varies considerably among the Verte- 

 brates, but to a lesser extent when the process is compared in closely 

 related species. The two layers to be distinguished are the ectoderm 

 and the endoderm. In many forms (e.g., the frog) the third germ layer, 

 or mesoderm, is formed almost simultaneously with the endoderm. 



Significance of the Germ Layers 



The distinction of germ layers, such as the ectoderm, mesoderm, 



and endoderm, is purely a matter of human convenience and is of no 

 real concern to the embryo. The mere fact that the endoderm and the 

 mesoderm are both derived originally from the ectoblast (outermost 

 layer of the blastula) suggests their fundamental similarity. By means 

 of experimental procedures it is possible to demonstrate that these 

 presumptive germ layers are interchangeable and that regions ordi- 

 narily destined to become, for example, brain (ectoderm), may be 

 transplanted to another region of the blastula where they will become 

 muscle (mesoderm) or possibly thyroid (endoderm). The germ layer 

 distinctions are based upon their position in the developing organism, 

 and their fate. The ultimate tissues of the adult are often classified, 

 again as a matter of human convenience, on the basis of groups which 

 are derived from one or another of these three primary germ layers. 

 The germ layers, which are first distinguishable with gastrulation, 

 therefore are definable by their fate in embryonic development. How- 

 ever, this fate is an arbitrary distinction since the fate of any group of 

 cells can be altered by transplantation. The importance, then, of this 



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