52 THE EMBRYOLOGY OF THE EYE. 



although the development of the iris changes 

 the relative positions. In cases of coloboma 

 of the iris Hess^ has in several cases found a 

 fibrous cord extending from the optic disc to 

 the posterior pole of the lens, and from this 

 point running meridionally to the equator of 

 the lens, where it was inserted into the ciliary 

 body at the head of the coloboma. This he 

 regards as a mesoblastic formation, which 

 passing in front of the margin of the secondary 

 vesicle has prevented the development of the 

 iris at this point. 



As is well known, in cases of incomplete or 

 arrested development of the eye, we usually 

 find remains of foetal vessels that have not 

 been absorbed. In my opinion, the cord that 

 Hess has described is a remnant of the foetal 

 vessels, which has remained simply because 

 the secondary vesicle has not developed ante- 

 -^riorly at some point, and the relations seen in 

 Fig. 4 are therefore preserved permanently — 

 in short, the persistent cord is not the cause 



' Grcefe's Archizi, xxxiv., 3. 



