PIGMENTATION OF LATERAL EYES OF VERTEBRATES 79 



pars iridica retinas, the outer layer being pigmented, the inner posterior 

 layer being at first destitute of pigment, but gradually becoming pig- 

 mented between the end of the third month and the seventh month. 

 The deposit commences at the margin of the pupil extending from the 

 anterior layer round the marginal sinus, and then outward in the posterior 

 layer as far as the bases of the ciliary processes ; beyond this the pigment 

 ceases to extend, and in the adult the posterior layer of the pars ciliaris 

 retinae is unpigmented. 



The mesodermal pigment of the choroid appears later than the ecto- 



mes. 



pl.ch.c. fe. 



•et. 



Fig. 51. — Section of the Retina of a 12-MM. Human Embryo. (R. J. G.) 

 p.g. : pigment granules lying in the epithelium which forms the outer 



wall of the optic cup. 

 ret. : retina. 

 pi. ch. c. : plexus chorio-capillaris, containing vessels filled with erythro- 



blasts. 

 mes. : mesoderm. 



dermal retinal pigment, about the beginning of the fifth month. The 

 pigment is found in irregularly branched cells — chromatophores — which 

 appear first in the outermost layer next the sclera and gradually spread 

 inwards. The same type of branched pigment cells are found in the 

 mesodermal layers of the ciliary region and iris. The development of 

 pigment in the stroma of the iris appears after birth and to a very variable 

 extent, being scanty in blue and grey eyes, but in brown eyes there is a 

 thick deposit in the superficial layers which obscures the view of the 

 vessels of the iris. In albinos pigment is diminished or absent in both the 

 retinal epithelium and the mesodermal layers of the eye. 



