layer the origin of the vitreous body. Seholer, therefore, must 

 be credited with the enunciation of the theory which later was to 

 be known as the Theory of the Mesodermic Origin of the Vitreous 

 Body. 



This theory became more generally known when R. Virehow, 

 in 1852, reviewed Seholer 's work, approved it, and classified the 

 vitreous body, which had always defied classification, as a special 

 kind of connective tissue. A. v. KoUiker, in 1861, also accepted the 

 new theory, and his authority blazed the way for Seholer 's views, 

 which thenceforth dominated biological circles almost to the end of 

 the last century and which find defenders even now. 



While all these authors agree as to the mesodermic origin of the 

 vitreous body, they differ widely in determining more precisely 

 the portion of the mesoblast genetically responsible for this tissue. 

 Most of them, following Seholer, derive the vitreous body from 

 the mesoderm entering the optic cup through the choroid fissure. 

 This view seems to be based largely on the observations made in the 

 development of the chick 's eye, in which other mesoblastic elements 

 are very scarce. Other investigators have called attention to the 

 relatively large circular opening between the lens and the inner 

 layer of the retina and the continuity of the extra-ocular mesen- 

 chyme with the vitreous body. Thus Van Pee attributes the origin 

 of a portion of the vitreous fibers to the mesenchyme surrounding the 

 optic cup on all sides and entering through the perilenticular open- 

 ing. But the most conflicting statements are made by the defend- 

 ers of the mesodermic origin regarding the very thin layer of mesen- 

 chyme originally found between the optic vesicle and the body ecto- 

 derm. Kolliker at first thought it probable that during the forma- 

 tion of the lens a part of this mesoderm is carried into the optic 

 cup and gives rise to the tunica vasculosa lentis. Later, however, 

 he began to doubt the correctness of this view. Arnold is more em- 

 phatic in attributing the origin of the vitreous body exclusively to 

 this portion of the mesoderm. On the other hand, Cirincione, the 

 most ardent defender of the mesodermic origin of the vitreous 

 body, rejects these explanations and returns to Seholer 's more gen- 

 erally accepted views. 



The first to challenge the theory of the mesodermic origin of the 

 vitreous body was L. Kessler, who, in 1871, made an exhaustive study 

 of the development of the chick's eye, 'and came to the conclusion 

 that the vitreous body is an amorphous gelatinous mass, formed as 



