ORGANS OF VISION OF THE VERT ERR AT A. 



415 



through the vitreous humour (tig. 295), and when it comes in close 



r.t 



a. c. r 



FIG. 295. SECTION THROUGH THE EYE OF A BABBIT EMBRYO OF ABOUT TWELVE DAYS. 



c. epithelium of cornea ; I. lens; mec. mesoblast growing in from the side to form 

 the cornea; rt. retina; a.c.r. arteria centralis retinas; of.n. optic nerve. 



The figure shews (1) the absence at this stage of mesoblast between the lens and 

 the epiblast : the interval between the two has however been made too great ; (2) the 

 arteria centralis retina? forming the vascular capsule of the lens and continuous with 

 vascular structures round the edges of the optic cup. 



proximity to the lens it divides into a number of radiating branches, which 

 pass round the edge of the lens, and form a vascular sheath which is 

 prolonged so as to cover the anterior wall of the lens. In front of the 

 lens they anastomose with vessels, coming from the iris, many of which 

 are venous (fig. 295) and the whole of the blood from the arteria centialis 

 is carried away by these veins. The vascular sheath surrounding the lens 

 receives the name of the membrana capsulo-pu pillar is. The posterior 

 part of it appears (Kessler, No. 372) to be formed of vessels without the 

 addition of any other structures, and is either formed simply by branches 

 of the arteria centralis, or out of the mesoblast cells involuted with the 

 lens. The anterior part of the vascular sheath is however inclosed in a 

 very delicate membrane, the membrana pupillaris, continuous at the sides 

 with the epithelium of Descemet's membrane. On the formation of the 

 iris this membrane lies superficially to it, and forms a kind of continuation 

 of the mesoblast of the iris over the front of the lens. 



The origin of this membrane is much disputed. By Kessler, whose 

 statements have been in the main followed, it is believed to appear com- 

 paratively late as an ingrowth of the stronia of the iris ; while Kolliker 

 believes it to be derived from a mesoblastic ingrowth between the front 

 wall of the lens and the epiblast. According to Kolliker this ingrowth 

 subsequently becomes split into two lamina?, one of which forms the cornea, 



