134 



INDUCTION AND ORGANISATION 



combination, the cristatus lens is too big for the taeniatus eye- 

 cup (PI. IX b-c). No continuous activity of the eye-cup is 

 necessary for the final differentiation of the lens. If an eye- 

 vesicle is removed after it has influenced 

 the ectoderm for 24 hours, the lens 

 primordium, as yet consisting only of a 

 slight thickening of the ectoderm, will go 

 on differentiating independently (Fila- 

 tow, 1934). The internal wall of the lens- 

 vesicle always supplies the lens fibres, 

 and its external wall the lens epithelium. 

 This polarity of the lens, too, is de- 

 termined by the eye-cup. However, other 

 organs than the eye-cup can exert the 

 same influence. If the lens-vesicle lies 

 in an abnormal part of the head, and 

 is in contact with an ear-vesicle, an 

 olfactory pit, or with the brain, lens 

 fibres are formed at the place of contact 

 (Balinsky, 1930; Dragomirow). Apart 

 from its polarity, the lens also has a 

 bilateral structure. Woerdeman (1939- 

 41) has shown that the various process- 

 es which play a part in the normal de- 

 velopment of the lens, such as the 

 formation of the lens-placode, the in- 

 vagination of the lens-vesicle, fibre 

 differentiation, and the development of 

 polarity and bilateral symmetry are 

 determined not simultaneously, but 

 successively. 



The induction which gives rise to a 

 lens is not species-specific. Not only in 

 heteroplastic, but even in "xenoplastic" transplantations a lens 

 may be induced, e.g., by an anuran eye-vesicle in the ectoderm 

 of a urodele, and vice versa (Holtfreter, 1935). 



It appears from an investigation on chick embryos by 

 McKeehan (1951) that direct contact of the eye-vesicle with 



Pig. 50. The size of 

 the lens depends on 

 that of the eye-cup. 

 Left: small eye-cups 

 of Bombinator, ob- 

 tained by the removal 

 of varying quantities 

 of material. Right: 

 the normal eye-cups 

 of the other half of 

 the body. After Spe- 

 mann. 



