144 THE THIRD DAY. [CHAP. 



Filling up a large part of the hollow of the optic cup is seen 

 a hyaline mass forming the hyaloid membrane and the coagulum 

 of the vitreous humour. In the neighbourhood of the lens it 

 seems to be continuous as at cl with the tissue a, which in turn 

 is continuous with the mesoblast m, and appears to be the 

 rudiment of the capsule of the lens and suspensory ligament. 



Thus while the hind moiety of the optic cup be- 

 comes the retina proper, including the choroid-pigment 

 in which the rods and cones are imbedded, the front 

 moiety is converted into the ciliary portion of the 

 retina, covering the ciliary processes, and into the uvea 

 of the iris ; the bodies of the ciliary processes and the 

 substance of the iris, their vessels, muscles, connective 

 tissue and ramified pigment, being derived from the 

 mesoblastic choroid. The margin of the pupil marks 

 the extreme lip of the optic vesicle, where the outer or 

 posterior wall turns round to join the inner or anterior. 



The ciliary muscle and the ligamentum pectinatum 

 are both derived from the mesoblast between the 

 cornea and the iris. 



The retina. At first, as we have said, the two walls 

 of the optic cup do not greatly differ in thickness. On 

 the third day the outer or posterioi becomes much 

 thinner than the inner or anterior, and by the middle 

 of the fourth day is reduced to a single layer of flat- 

 tened cells (Fig. 51, p. Gh.). At about the 80th hour 

 its cells commence to receive a deposit of pigment, and 

 eventually form the so-called pigmentary epithelium of 

 the choroid ; from them no part of the true retina (or 

 no other part of the retina, if the pigment-layer in 

 question be supposed to belong more truly to the retina 

 than to the choroid) is derived. 



