V.] THE RETINA. 105 



the formation of the vitreous humour within the cup, the 

 edges of the slit grow completely together, and all traces of 

 the seam disappear. In birds the course of events is some- 

 what different. 



In addition to such amount of mesoblast as may pass 

 through the slit to form the vitreous humour, two special 

 processes of mesoblast grow in, one in the neighbourhood of 

 the optic stalk, in the region of the true retina, and a second, 

 which speedily becomes highly vascular, in that portion of 

 the slit which corresponds to the ciliary part of the retina. 

 The former piocess remains as the pecten so characteristic 

 of the avian eye, while the latter vascular process serves to 

 supply the pecten with blood. 



By the twelfth day the fissure completely closes up and 

 disappears between these two processes and also in front of 

 the vascular one; but both the pecten and the vascular process 

 are left projecting into the interior of the eye. Hence in the 

 adult eye, the pecten seems to perforate the retina close to 

 the entrance of the optic nerve, the nervous fibres of the 

 retina spreading away in a radiate manner from it. 



The optic stalk, which, as we have said, by an obliteration 

 of its central canal becomes converted into the optic nerve, is 

 at first equally continuous with the inner and with the outer 

 wall of the retina. This must of necessity be the case, since 

 the interval which primarily exists between the two walls 

 is continuous with the cavity of the stalk (vide Figs. 28 

 and 30 F, s). When the fibres however make their appear- 

 ance in the nerve, they are found to be connected with the 

 inner wall, or functional retina, only. 



The histological condition of the eye in its earliest 

 stages is very simple. Both the epiblast forming the walls of 

 the optic vesicle, and the superficial layer which is thickened 

 to become the lens, are composed of several layers of simple 

 columnar cells. The surrounding mesoblast is made up of 

 cells whose protoplasm is more or less branched and irregu- 

 lar. These simple elements are gradually modified into the 

 complicated tissues of the adult eye, the changes undergone 

 being most marked in the cases of the retina, the optic nerve, 

 and the lens with its appendages. 



The retina. At first the two walls of the optic cup do not 

 greatly differ in thickness. On the third day the outer or 



