414 



THE EYE IN EVOLUTION 



The blue jay 

 CyunoclUa 



all the time, however, close to the bulbar wall and not penetrating 

 far into the vitreous, such as in the pigeon, Coluniba, and the herring 

 gull, Larus argentatus (Figs. 515-8) ; 



(3) a slender sickle-shaped structure proceeding with a curved 

 course from the disc towards the equator of the lens, such as in the 

 blue jay, Cyanocitta cristata, and the chimney swallow, Hirundo rustica 

 (Figs. 519-20), sometimes almost touching it, as in the Anseriform birds 

 (goose, swan). Between these last two forms gradations occur, such as 

 is seen in the great spotted woodpecker, Dendrocopus major ; 



The swallow 

 Hirundo 





The woodpecker 



Dendrocopus 



major 



Fig. ,521. — The Microscopic Structure of the Pecten of the Chicken 

 { X 84) (Norman Ashton). 



(4) a cone-shaped structure without pleats, uniquely found in the 

 kiwi, Apteryx (Plate XII, Fig. 1 ; Fig. 512). 



The histological structure of the pecten has received much atten- 

 tion (Fig. 521).i Essentially it is made up of a dense and elaborate 

 capillary network associated with a comparatively small amount of 

 supporting tissue ; this was originally (Mihalkovics, 1873 ; Leuckart, 

 1876 ; Kessler, 1877) and sometimes has subsequently (Bacsich and 

 Gellert, 1935) been said to be mesodermal in origin, but following the 

 work of Bernd (1905) and Franz (1908), has been generally accepted to 

 be glial in nature. The glial tissue derived from the optic disc is more of 

 the nature of a syncytium than cellular. The rich vascular plexus, 

 \-, liich is composed of vessels of greater than capillary size, is supplied 



1 Mihalkovics ( 1 873), Denissenko (1881), Bernd (1905), Franz (1908-9), Blochmann 

 : V. Husen (1911), Lschreyt (1914), Kajikawa (1923), Mann (1924), Menner (1935), 

 ika (1938). 



