18 



prive the retina of its controlling influence in the further develop- 

 ment of the vitreous body, which throughout remains functionally 

 dependent on it. The vitreous body, therefore, at this stage of de- 

 velopment is a very complex structure, the constituent parts of 

 which are partly ectodermal and partly mesodermal in their origin. 





 III THE PERMANENT VITREOUS BODY 



The structure of -the vitreous body described in the preceding 

 chapter, is not a permanent one. The hyaloid vascular system, hav- 

 ing fulfilled its function of supplying nourishment to the crystalline 

 lens, soon shows signs of degeneration and gradually disappears. 

 The remnants of the mesodermal cells begin to disintegrate and 

 dissolve. The vitreous body then appears again in its original 

 purity, modified indeed and greatly changed, but in its essentials 

 like the primitive ectodermal vitreous body. An excellent repre- 

 sentation of its general appearance and internal structure is found 

 on plate VI, showing various portions of the eye of a fetus 220 mm 

 in length. 



Figure 17 represents . a horizontal section of the eye, passing 

 through the optic nerve and the lens in a plane parallel to the hya- 

 loid artery, and bisecting the hyaloid canal. The lens has been 

 slightly displaced in the act of sectioning, causing some disarrange- 

 ment of the adjacent structures, especially of the fibers of the 

 zonula ciliaris. The vitreous body, however, is in perfect condition. 

 The section was stained in H eld's molybdic haematoxylin, which 

 shows very clearly the delicate fibers, but unfortunately overstains 

 the remaining structures. The small crystals found scattered here 

 and there are owing to mercuric chloride used in killing and fixing 

 the material. 



The salient feature of the vitreous body at this period of de- 

 velopment, compared with figure 5, is the absence of the large meso- 

 dermal ingredient of the previous stages of development. All that 

 remains of the extensive hyaloid arterial system is the main trunk, 

 the hyaloid artery, extending through the entire posterior chamber 

 of the eye, from the optic nerve to the lens, to which it still ad- 

 heres. Its various branches have already disappeared. A closer 

 examination of the vitreous body, however, reveals a large number 

 of cells in various stages of disintegration. The methods used in the 

 preparation of the material and the heavy stain make it impossible 



