602 TEXT-BOOK OF EMBRYOLOGY. 



PRACTICAL SUGGESTIONS. 



THE EYE. 



Embryos of the chick, pig and rabbit furnish excellent and easily available material for 

 the study of the development of the eye. They should be fixed in Zenker's or Bouin's fluid, 

 embedded in paraffin, sectioned transversely through the proper levels, stained with Weigert's 

 haematoxylin and eosin, and mounted in xylol-damar. 



The chick embryo of twenty-four to twenty-eight hours' incubation shows the three 

 primary brain vesicles and the optic evaginations. 



At thirty-six hours' incubation, the evagination has become a distinct vesicle connected 

 with the brain by the optic stalk. The outer wall of the vesicle lies just under th surface 

 ectoderm, which is here thickened to form the lens area. 



At about forty-eight hours the lens area has changed to a distinct invagination and the 

 optic vesicle has become flattened. 



At from fifty -four to fifty-eight hours the lens invagination has become much larger, and 

 its lips have begun to approximate. The optic cup is at this stage shallow and its two layers 

 are in apposition at the bottom of the cup, while at the edges of the cup the remains of the 

 original cavity of the vesicle are still visible. 



At about eighty-five hours a distinct lens vesicle has been formed, which is completely 

 separated from the surface ectoderm. 



At about ninety-five hours a definite lens has been formed with long nucleated lens 

 fibers and an anterior epithelium. The outer layer of the optic cup begins to show pigment 

 granules. 



Should human embryos be available, the following data may be of service: 



At what has been estimated as about four weeks, the lens is formed and lies in the mouth 

 of the optic cup. It is separated by mesoderm from the surface ectoderm (epithelium of the 

 cornea). The optic cup shows two distinct layers the outer of which contains pigment 

 granules. The cavity of the vitreous shows, as does also a layer of vascular mesoderm 

 surrounding the cup the anlage of the chorioid. 



At from five to six weeks the mesodermal anlagen of the substantia propria corneas and 

 of the tunica vasculosa lends can be seen. The vitreous is more distinctly formed than in 

 the preceding. 



At from seven to eight weeks the lens shows distinct lens fibers and anterior lens epithe- 

 lium. There is also a beginning differentiation between the chorioid and sclera. 



At from nine to ten weeks the mesodermal vessels have extended into the vitreous, both 

 arteria centralis retinae and its continuation into the vitreous as the hyaloid artery having 

 developed. Nerve fibers can be seen in the periphery of the optic stalk. These can be 

 traced to the nerve cells of the ganglion cell layer of the retina. 



At eleven to twelve weeks the corneal epithelium shows distinctly and the substantia 

 propria corneae has become differentiated by the formation of the anterior chamber. The 

 eyelid is developed to the edge of the lens. 



At from thirteen to fifteen weeks the eyelids become closed with the formation of a distinct 

 conjunct! val sac. The chorioid and sclera are more completely differentiated. There are 

 also present the anlagen of the lacrymal glands, of the Meibomian glands and of the tarsus. 



At sixteen to seventeen weeks the ciliary body and processes begin to appear; also the 

 iris, the differentiation between the pars optica retinae and the pars ciliaris and pars iridicae 

 retinae, the membrana pupillaris and the lens capsule. The anterior chamber has also 

 become much more distinct. 



