136 INDUCTION AND ORGANISATION 



the eye-vesicles, the lateral walls of the neural tube come into 

 very close contact with the ectoderm. Later this part of the 

 brain will form a paired protusion, the cerebral hemispheres. 

 The ectoderm which is in contact with these parts of the brain 

 wall thickens into a pair of olfactory placodes, which later, by 

 invagination, form the olfactory pits. Several experiments 

 have shown that this differentiation of the ectoderm is also 

 caused by inductions from the brain wall (Raven, 1933). If 

 the rostral end of the brain is double, either partly, or com- 

 pletely, an olfactory pit is formed in contact with each of the 

 cerebral hemispheres. If one hemisphere is absent, the corres- 

 ponding olfactory pit is absent, too. In cyclopic embryos, the 

 fore-brain is poorly developed because of deficiencies in the 

 anterior part of the archenteron roof. In such cases, there is 

 not only a single median eye, but also a single olfactory pit, 

 situated in front of the anterior end of the brain. This, again, 

 is a non-specific induction. Holtfreter (1935) found that, in the 

 xenoplastic combination of anuran with urodele cell material, 

 induction of olfactory pits was still possible. 



In the case of the olfactory organs, the stage at which the 

 definitive determination by contact induction takes place is 

 probably preceded by a phase during which there is a slight, 

 labile "predetermination" already, just as we have seen in the 

 case of the lens of the eye (above, p. 132). In particular in some 

 anurans, this early phase of determination may be very im- 

 portant, and can replace induction by the fore-brain completely, 

 or nearly so (Zwilling, 1940) ; it probably originates under the 

 influence of the archenteron roof. 



The auditory organ, too, arises from an ectodermal thicken- 

 ing, the ear-placodCj which invaginates and becomes separated 

 from the ectoderm, forming the ear-vesicle. This gives rise 

 to the various parts of the internal ear by a series of folding 

 processes. In urodeles the area concerned generally acquires 

 an autonomous capacity to form ear-vesicle during the closing 

 of the neural tube. In anurans this may happen a little earlier. 

 Yntema (1939), however, has shown that in Amblystoma 

 punctatum a first, weak determination takes place at a much 

 earlier stage, viz. at the late gastrula stage. 



