A MAP OF THE EARLY GASTRULA 61 



Let us diagram an early gastrula and sketch an embryo with a few of the 

 main structures, such as the primitive nervous system, the notochord, the 

 digestive tract, and the heart (Fig. 25). Then by staining regions in the early 

 gastrula it is possible to find out where these stained areas appear in the 

 embryo. In the first place, if we stain region a above the dorsal lip with Nile 

 blue sulfate this region will be found in the anterior end of the gut, and thus 

 region a forms head endoderm. If we use another dye, a red dye this time, 

 and stain region b, we find that this dye is present in the notochord of the 

 embryo. These cells on the surface of the gastrula somehow get into the 

 interior of the embryo during normal development and split off to form 

 notochord. 



Let us continue this analysis and stain another region, c, of the gastrula 

 and follow it through development into the embryo. Region c will be located 

 in the nervous system. Continuing in a counterclockwise fashion, we may 

 alternate red and blue stains, and region d near the animal pole of the gastrula 

 will form a large part of the brain. Continuing further, region e will be 

 found in the head epidermis of the embryo; / in the ventral epidermis; g in 

 the posterior epidermis; and b and i in the posterior and anterior regions of 

 the gut, respectively. 



In this way all the regions in the gastrula can be mapped out with respect 

 to their future development. This procedure is called the mapping of the 

 presumptive values of the parts of the gastrula. The presumptive value of a 

 part is simply that structure or region which the part becomes in the course 

 of normal development. The emphasis is on normal development because it 

 will be shown that under experimental conditions parts can develop differ- 

 ently from the way they do normally. 



Figure 26 presents a gastrula from the side view and a gastrula viewed 

 from the dorsal lip. In side view we see a sphere in which, first of all, we have 

 drawn a line approximately bisecting the sphere, giving one hemisphere 

 which forms ectodermal derivatives, the presumptive neural plate, and the 

 presumptive epidermis, and another hemisphere which gives rise to meso- 

 dermal and endodermal derivatives. 



Within each of these areas further division has been made so that a line 

 drawn from near the animal pole to the boundary of the mesoderm delineates 

 a region which, as we have seen by vital staining, forms the nervous system 

 and is therefore called presumptive neural plate — presumptive because with 

 development continuing as normal it is presumed to form neural plate and 



