GASTRULATION AS A CRITICAL STAGE IN DEVELOPMENT 101 



Epidermis 



Sucker 



Sucker 



Epidermis 



Lens 



Neural fold 

 Ear 

 Limit of head region 



Limit of inturned 

 material 



Notochord 

 Mesoderm, somites 

 Anterior limb bud 



Blastopore lip 

 Visceral pouches 



POSTERIOR- DORSAL 

 VIEW 



RIGHT LATERAL 

 VIEW 



Fate map showing presumptive regions of the anuran blastula. (Adapted from 



Vogt: 1929.) 



frog blastula. However, the fate maps of closely related species have 

 been worked out and, combined with circumstantial experimental 

 evidence on the frog's egg, we are able to supply what is believed to be 

 a reasonably accurate fate map of the frog blastula. 



Under conditions of normal development, the ectoderm of the frog 

 is derived in part from the cells of the animal hemisphere and in part 

 from the intermediate or equatorial plate cells. These are the regions 

 on the fate map designated as presumptive ectoderm. The endoderm 

 comes partly from the intermediate zone but largely from the vegetal 

 hemisphere area (i.e., the presumptive endoderm on the fate map). 

 The mesoderm and the notochord have a dual origin, arising between 

 the ectoderm and the endoderm, largely from the region known as the 

 lips of the blastopore. The student is advised to study carefully the 

 accompanying fate maps. 



Gastrulation as a Critical Stage in Development 



One of the reasons for the fact that gastrulation is such a critical 

 stage in embryonic development is that not only do the cells divide but 

 also various cell areas take on special significance which they did not 

 previously have. This special significance is shown by the mere fact 

 that fate maps have been derived. There is a new independence, as 

 well as interdependence, of various areas of the embryo. This change is 

 designated as differentiation. Instead of a group of somewhat similar 



