67 EMBRYOLOGY 



finally the nervous system. The remainder of the ectodermal hemisphere is 

 presumptive epidermis. This forms the entire outer covering of the embryo, 

 the epidermal layer of the skin. The cells giving rise to endoderm are out- 

 lined by a region just above the dorsal lip of the blastopore and including 

 all the material near the vegetal pole. This region is presumptive endoderm, 

 which will form the lining of the gut and digestive glands. 



Between the presumptive ectodermal structures and the presumptive 

 endoderm there is an intermediate region in which we distinguish, first of 

 all, the presumptive notochord. The notochord is derived from a plate of 

 cells above the dorsal lip, and becomes a long, narrow rod in the embryo. 

 This was shown to be true by the experiment in which region b was stained 

 and followed directly into the notochord of the embryo. The remainder of 

 the gastrula forms mesoderm. The presumptive mesodermal region is further 

 subdivided into an area which forms the somites and mesoderm which forms 

 the lateral plate, or lateral mesoderm. The somites are blocks of mesoderm 

 which develop into the muscles of the back and other structures to be dis- 

 cussed later. 



Thus, in a general way these areas account for all the cells of the gastrula 

 and give rise to all the parts of the embryo. Such plans are called gastrular 

 maps of presumptive values. Maps of different species vary in the size and 

 exact boundaries of presumptive structures, and Figure 26 simply gives gen- 

 eral relationships. 



With these maps as a guide it is possible to look at the living gastrula 

 (Fig. 27) and know the fate of the cells in later development. The curved 

 lip of the blastopore serves as a basis of orientation for locating the pre- 

 sumptive structures. Thus in the early gastrula one is able to transplant parts 

 having a known fate. A part may also be cut out and cultivated in salt 

 solutions to see how it will develop when separated from neighboring 

 parts. 



Gastrular maps have been constructed for the fish, frog, salamander, 

 and chick, and they show a similarity of pattern. In the case of the frog the 

 presumptive structures of the embryo are located on the external surface of a 

 sphere. Presumptive structures such as the heart, liver, kidney, and digestive 

 tract are externally located in the early gastrula stage. In a serlse these struc- 

 tures are turned "inside out." Presumptive areas on the gastrula have much 

 different shapes than the actual structure in the embryo. Whereas the pre- 

 sumptive neural plate is a short, broad plate of cells, the actual neural tube 



