THE POTENCIES OF THE ECTODERM OF AN EARLY GASTRULA 75 



esting point should be noted here: the forelimb is composed largely of 

 mesoderm, but presumptive eye is ectoderm, and thus an ectodermal struc- 

 ture has been converted into mesoderm. Also, cartilage is a typical mesodermal 

 derivative, and yet eye ectoderm can form cartilage. Therefore the germ 

 layers are not irreversibly determined in early development. 



In the flank region (3) a cross section through the tissues of the trans- 

 plant shows, in addition to the normal somites, neural tube, pronephric duct, 

 and notochord of the host, an almost exact duplicate of these structures in 

 the transplant (Fig. 32). The latter forms a secondary neural tube accom- 

 panied by a secondary notochord, a large block of mesoderm resembling 

 a somite, and a cross section of a tube which proves to be the pronephric 

 duct. Thus the presumptive eye tissue when placed in the environment of 

 the flank structures develops into flank structures. And now we may add to 

 our list of potencies notochord, spinal cord, somites, and pronephros. 



Finally, if this presumptive eye tissue is explanted to a fourth environ- 

 ment, a salt solution — if it is simply cut out and allowed to develop in a salt 

 solution — it forms no recognizable structures whatsoever (Fig. 32). In a 

 salt solution the isolated presumptive eye becomes an irregular mass of cells 

 and finally dies. Thus it has no capacity of its own to develop into any one 

 structure, for if it is put into a salt solution away from the influences of the 

 rest of the embryo it forms nothing. In its normal position the presumptive 

 eye does form eye, but in new positions it can form anything; indeed a 

 thorough analysis of the transplantations of the eye ectoderm of the early 

 gastrula shows that it can form almost any structure in the body. 



The same analysis may be repeated with any region in the ectodermal 

 hemisphere of the early gastrula. The presumptive epidermis, if tested in 

 the same way, gives the same results as the presumptive eye. Therefore the 

 ectoderm of the early gastrula is truly undifferentiated tissue. It can be 

 molded or induced to form almost any structure in the body. 



It is important to recognize, however, that this undifferentiated state is 

 true for the ectoderm of the early gastrula only. The ectoderm has many 

 potencies at that time. At some later time — in the neurula stage (Fig. 33), 

 for example — the same eye tissue has only one potency. That is, it will form 

 only eye. Between the beginning of gastrulation and the neurula stage, there- 

 fore, some process has reduced the several potencies of the early gastrula to 

 a single potency in the neurula. 



