98 GASTRULATION 



distinction is one of position, and the ectoderm is always the outer- 

 most layer of cells, the endoderm the innermost, and the mesoderm 

 the intermediate layer of cells present when gastrulation has been com- 

 pleted. 



In recapitulation, a histologist could define the ectoderm as that 

 group of embryonic cells which, under normal conditions of develop- 

 ment, give rise to the epidermis and the epidermal structures, to the 

 entire nervous system and sense organs, to the stomodeum, and to the 

 proctodeum. The endoderm would be defined as that group of em- 

 bryonic cells which normally give rise to the lining epithelium of the 

 entire alimentary tract and all of its outgrowths, such as the thyroid, 

 lungs, liver, pancreas, etc. The mesoderm would be defined as that 

 group of cells which normally give rise to the skeleton and connective 

 tissues, muscles, blood and vascular systems, to the coelomic epi- 

 thelium and its derivatives, and to most of the urogenital system. The 

 notochord in the frog is derived simultaneously with the mesoderm, 

 and probably from the dorsal lip epiblast. 



It is important, therefore, for the student to avoid ascribing any 

 peculiar powers to the germ layers as such. They have their particular 

 fate in development not by virtue of any peculiarly endowed power 

 but rather by their position in the developing embryo as a whole. 



Origin of Fate Maps 



Gastrulation is a critical stage in the development of any embryo. 

 This is due in part to the fact that the positional relationship of the 

 various cells of the late blastula and early gastrula begins to take on 

 special significance. This has been demonstrated by many experimental 

 embryologists, among whom are His, Born, BUtschli, Rhumbler, Spek, 

 Vogt, Dalcq, Pasteels, Vintemberger, Holtfreter, Nicholas, and 

 Schechtman. They have used various experimental devices, such as 

 injury or excision of cell areas, or staining local areas with vital dyes 

 and following their subsequent movement. By such methods the so- 

 called "fate maps" of various blastulae have been worked out. 



A fate map is simply a topographical surface mapping of the blastula 

 with respect to the ultimate fate of the various areas. When one traces 

 a cell group on the surface of a blastula which has been vitally stained 

 with Nile blue sulfate, for instance, and finds that these cells move 

 from the marginal zone (between the animal and the vegetal hemi- 

 spheres) over the dorsal lip of the early blastopore and into the embryo. 



