72 EMBRYOLOGY 



through the neural crests which give rise to the spinal ganglia. Underneath 

 the spinal cord are the notochord and dorsal aorta. Flanking the spinal cord 

 are the paired somites, which differentiate into muscle of the back, cartilage 

 of the vertebrae, and connective tissue of the skin. Below the somites the 

 mesoderm forms undifferentiated kidney tissue or nephrotome. The mes- 

 onephros, or permanent kidney, differentiates from the nephrotome in later 

 development. The rest of the mesoderm forms the somatic and splanchnic 

 layers which enclose the coelom. The somatic layer of the mesoderm develops 

 in part into the limbs. The splanchnic layer gives rise chiefly to the lining of 

 the body cavity, and to mesenteries which support the viscera. 



The potencies of the ectoderm of an 

 early gastrula 



The outline given above completes the necessary survey of structures, and 

 we now proceed with our analysis of potencies. We shall begin by choosing 

 a very definite region, such as the presumptive eye region. Figure 32 shows 

 the way in which the tests are carried out and the results obtained. As 

 previously noted, approximately half of the early gastrula is ectoderm. One 

 spherical sector of it is destined to become neural plate, and this sector is 

 called presumptive neural plate. The remainder is called presumptive epi- 

 dermis. The dorsal lip of the blastopore will serve to mark the hemisphere 

 which gives rise to endoderm and mesoderm. 



The part we choose to test is a region very close to the animal pole of 

 the blastula. We may test the presumptive value of this region by staining 

 it with Nile blue sulfate and following the development of the stained 

 region into the later embryo. The stained region will appear in the eye, and 

 therefore its presumptive value is eye. During normal development the cells 

 in this region form the optic cup and optic stalk. In order to test the potencies 

 of the presumptive eye region — that is, its abilities to differentiate into other 

 types of tissue — we shall transplant it into various positions in older embryos. 

 The embryo receiving the transplant is one in which structures such as the 

 eye and the brain, the heart, the gills, and pronephros are already forming. 

 Thus the transplanted cells will be exposed to the action of differentiating 

 structures. We cut out three presumptive eye regions with glass needles and 



