2 THE EMBRYOLOGY OF THE EYE. 



tion of what I have myself seen. When some 

 essential point has not been shown in my 

 specimens, an authority that has described it 

 is quoted. In regard to some disputed points 

 the views held by different authorities are given. 

 The earliest periods up to the formation of 

 the lens-sac have been studied in the embryo- 

 chick, and the later periods, on account of the 

 difficulty in obtaining a complete series of 

 human embryos, have been studied in the 

 foetal rabbit and pig, the eye of the latter 

 closely resembling that of man. 



I. — A Sketch of the Processes Occurring. 



At an early stage the ovum consists exter- 

 nally of a layer of closely packed cells having a 

 definite form and arrangement (the epiblast), 

 and beneath or internal to the epiblast of a 

 layer of loosely connected branched cells of 

 irregular form (the mesoblast). As the hypo- 

 blast does not take part in the development 

 of the eye, it need not be spoken of here. 



Very early in the development of the epi- 



