II. THE PERIOD OF ORGAN DEVELOPMENT 131 



parts of the rostral end of the neural plate were transplanted 

 into other areas of the germ have shown that such grafts 

 could develop into normal eyes (Fig. 48). Evidently, the potency 

 for the formation of an ej^e-vesicle is already present in the 

 anterior parts of the newly formed neural plate. Mangold 

 (1928-29) has even shown that the material concerned possess- 

 es this eye-forming potency as early as the late gastrula stage, 

 before there is any neural plate at all. The ectoderm acquires 

 this potency under the influence of induction by the anterior 

 part of the archenteron roof. Experiments by Spemann (1931) 

 have shown that, after transplantation into the ventral side 

 of another embryo, this part of the organisation centre induces 

 not only brain tissue, but eyes as well (PL VII b). It is note- 

 worthy that the eye-forming potency is at first evenly dis- 

 tributed throughout the anterior region of the neural plate, 

 so that any part of it will form an eye after transplantation. 

 It is not until later that the eye-forming power becomes con- 

 centrated in two lateral centres, and lost in the median region. 

 Experiments by Adelmann (1929-37) have shown that this, 

 too, is due to the activity of the archenteron roof. From a 

 certain stage onwards, a narrow median strip of the archenteron 

 roof begins to exert an induction which suppresses the eye- 

 forming potency in the overlying ectoderm. This observation 

 supplies the explanation of a common deformity: chemical or 

 mechanical injury to the median part of the archenteron roof 

 in the head region destroys its inhibitory influence, and there- 

 fore, instead of two lateral eyes, a single median eye now 

 arises from the bottom of the neural tube (so-called cyclopia). 

 The next problem is whether, simultaneously with the pro- 

 trusion of the eye-vesicle, its later differentiation into retina, 

 pigment epithelium, and eye-stalk is also unequivocally deter- 

 mined. A grafted eye-primordium invaginates in the normal 

 manner, and forms an eye-cup. The differentiation of retina 

 and pigment epithelium also takes its normal course. This 

 shows that the determining influence of the archenteron roof 

 has also to some extent fixed the later phases of the develop- 

 ment of the eye-cup. Experiments by Dragomirow (1932-37), 

 however, have shown that, at the eye-vesicle stage, parts of the 



