ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 303 



network is established, the vitelline circulation becomes partially arterial, 

 in virtue of the development of the mesenteric artery and its vitelline 

 branches. There is a marked asymmetry in the vitelline circulation ; 

 bilateral symmetry is not established until the absorption of the yolk is 

 completed. The primary circulation in the stickleback is very like the 

 primitive circulation of Amphioxus, though the stickleback is far from 

 being archaic. J. A. T. 



Development of Shoulder-Girdle of Pig. — F. B. Hanson {Anat. 

 Record, 1920, 18, 1-21, 28 figs.). There is a permanent supra-scapula, 

 in which no centres of ossification appear. In the entire shoulder- 

 girdle there are only two ossific centres : one for the scapular l^lade and 

 one for the subcoracoid. Coracoid process, acromion, and clavicle are 

 aborted. The pig's scapula is at once primitive and degenerate. 



J. A. T. 



Eye-colour in Birds.— C. J. Bond {Journ. Genetics, 1919, 9, 

 69-81). An analysis of certain factors concerned in the production of 

 eye-colour in birds. The " bull " eye owes its black or dark colour to 

 the absence of pigment on the anterior surface of the iris. The delicacy 

 and translucency of the iris tissues allow the posterior uveal pigment to 

 shine through, and this gives an appearance of blackness. Another type 

 is the " pearl " eye, where anterior iris pigment is absent, but the iris 

 tissues are not translucent as in the "bull" eye. They are crowded 

 with granules which are themselves colourless, but prevent the passage 

 of transmitted light, and when seen by reflected light give a grey-white 

 appearance to the surface of the iris. Another type is the " yellow " or 

 " gravel " eye, due in the pigeon to a network of branching cells 

 crowded with small spherical yellow granules. These cells lie on the 

 anterior surface of the iris ; tiiey cover the capillary Ijlood vessels and 

 surround the striated muscle fibres of the iris, which in the pigeon are 

 themselves free from pigment. Amongst birds, as amongst mammals, 

 quite a large number of species possess anterior iris pigment which 

 passes through all grades of brown up to black. AYith some exceptions, 

 notably the Silky Fowl, the black iris is associated with black feather 

 colour, and in its deeper grades with black-leg colour. Histologically 

 the brown (in the darker shades) and the black iris are produced by a 

 •well-defined layer of characteristic branching cells, which contain dark 

 brown or black pigment, on the anterior surface of the iris. These cells 

 intercommunicate by their l)ranches and form a plexus of pigmented 

 cells thickest over the capillaries in the peripheral or middle zones of 

 the iris. The author deals also with the " black " eye in fowls, with 

 the "triplex" eye, containing two kinds of anterior iris pigment, the 

 " ruby " eye (due to a surface layer of branching cells with yellow pig- 

 ment, which surround the capillaries on the anterior surface of the iris), 

 and the "parti- or zone-" coloured iris. He contributes notes on the 

 genetics of the various factors. Some special cases of coloration are 

 analyzed. Thus in Lawes' Bird of Paradise a brilliant colour effect is 

 produced by the combination of three factors : (1) thinness and trans- 

 lucency of the iris tissue, allowing the uveal pigment to shine through ; 



