THE GENESIS OF THE EYE 



117 



of the skin become prtijected in the temperature receptors seen in vipers in which 

 a facial pit -like " eye " has developed for the reception of infra-red radiation 

 (Bullock et al., 1952-56).^ The eye, of course, has transcended all other organs 

 in this respect, projecting itself to astronomical distances and responding to a 

 few qvianta of luminous energy. 



Originally it is probable that within the main groups, or modalities, 

 appreciation was relatively undifferentiated ^ ; for example, a usual 

 accompaniment of the radio -receptors is an absorbing pigment, and it 

 is possible that the early pigmented cell responded to thermal energy as 

 well as to luminous radiation. Subsequent evolution, both in the receptors 

 and in their cerebral connections, determined not only an increase in 

 the number of modalities (touch, temperature, smell, sight, hearing) 

 but eventually led to the differentiation of various individual receptors 

 within the same modality, thus allowing the emergence of qualities 

 within a modality, such as colour within the modality of sight. 



PIGMENTS 



PIGMENT is a common feature of photoreceptors of all tyjDcs ; 

 indeed, Bernard (1S97) suggested that light-sensitive cells first arose as 

 modifications of the epidermis induced by crowding of pigment granules 

 in situations which were most frequently and brilliantly illuminated. 

 The physical energy of light can be converted into physiological 

 activity only in so far as it is absorbed, and the primary function of 

 the deposition of pigment in the neighbourhood of light-sensitive 

 areas is to serve as an absorbing agent ; a further development is 

 the initiation of a specific photochemical reaction. 



In its simplest form, pigment may aid the general dermal sensitivity 

 to light, a function well illustrated in Echinoderms. Thus the entire 

 surface of the sea-cucumber, Holothuria, is photosensitive and is 

 coloured by two greenish-yellow pigments ; the reaction of the animal 

 varies with the amount of pigment present, for young and lightly 

 pigmented individuals are poorly light-sensitive while heavily pig- 

 mented adults are markedly so (Crozier, 1914-15). Again, the sea- 

 urchin, Centrostephanus Joricjisinnus. shows a high light-sensitivity in 

 the violet spicules around the anal orifice whence a purple pigment can 

 be extracted (von Uexkiill. 1900) — an early example, incidentally, of the 

 frequent aggregation of sensory organs around the body orifices. 

 When, however, specific light-sensitive organs are developed, pigment 

 is concentrated in tlieir vicinity — melanin as an absorbent and 

 visual pigments as sensitizers. All these pigments are synthesized 

 by special cells called ciiromatoblasts {xpojyt-y.. colour ; ^Xxaros, a 

 sprout). 



1 See further, p. 600 « p. 109. 



Holothuria 



Cetitrostephanus ^- 



