THE POTENCIES OF THE ECTODERM OF AN EARLY GASTRULA 73 



transplant them into an older embryo in three different positions (Fig. 32). 



In the first experiment the presumptive eye is placed in the head region 

 (1) of the older embryo. In another experiment it is transplanted in the 

 region of the gills (2). And to provide a third environment, it is put in the 

 flank region (3). In these three regions this presumptive eye will come in 

 contact with foreign tissues. In region 1 it will come in contact with the 

 brain; in region 2, with the gills and forelimb; and in region 3, with the 

 pronephros, notochord, and somites. 



In each case the transplanted tissue fuses with the tissues of the host, 

 develops, and forms an outgrowth from the side of the embryo. If, at some 

 later stage, after some development has taken place, we cut through these 

 outgrowths and make histological sections through them, we are able to 

 identify the structures which have formed from the transplanted tissue. 

 Figure 32 shows such cross sections through regions 1, 2, and 3. A section 

 through region 1 passes through the brain and eye region of the host. How- 

 ever, in addition to the host's brain and eye we find in the transplanted tissue 

 another eye, a nasal placode, and a secondary brain. The structures are found 

 to be covered by epidermis which came from the transplant. 



It must be emphasized that all of these structures developed from the 

 presumptive eye tissue which was transplanted. The tissues of the host are 

 not affected by the transplantation and develop in a normal fashion. 



This result means that the presumptive eye of the gastrula — the part we 

 started with — has, under the influence of the head region of the host, 

 developed not only into eye but also into brain, nasal placode, and epidermis. 

 In addition to its normal ability to form an eye, therefore, it has at least 

 three other potencies for development. Let us make a list of the potencies as 

 we find them. Eye is the presumptive value. Brain, epidermis, and nasal 

 placode are additional potencies. 



Sections through the gill region (2) reveal additional potencies. A 

 section through the gill region shows the structures that are present in the 

 transplant; these are a primitive internal ear, or otocyst, a neural tube, 

 cartilage, and gills. A forelimb, which is partly composed of host tissue 

 and partly of donor tissue, is also obtained. 



Presumptive eye in the gill region, then, can form all the structures 

 characteristic of that region, and we may now add to the list of potencies of 

 the presumptive eye tissue otocyst, gills, forelimb, and cartilage. An inter- 



