486 EMBRYOLOGY. 



fibrous layer. The former, or the pial sheath, unites the pia mater 

 of the brain and the choroid membrane of the eye ; the latter, or the 

 dural sheath, is a continuation of the dura mater and at the eye- 

 ball becomes continuous with the sclerotica. Later the optic nerve 

 acquires a still more complicated structure, owing to the fact that 

 vascular processes of the pial sheath grow into it and provide the 

 nerve-bundles and the epithelial sustentative cells belonging to them 

 with connective-tissue investments. 



As has been previously stated, the direction in which optic fibres grow into 

 the stalk of the optic vesicle is still a subject of controversy. His, with whom 

 KOLLIKER is in agreement, maintains that they grow out from groups of gang- 

 lionic cells (thalamus opticus, corpora quadrigemina), and are only secondarily 

 distributed in the retina. He supports his view on the one hand by the agree- 

 ment in this particular which exists with the development of the remaining 

 peripheral nerves, and on the other by the circumstance that the nerve-fibres 

 are first distinctly recognisable in the vicinity of the brain. 



W. MtJLLEB, on the contrary, believes that the outgrowth takes place in the 

 opposite direction ; he maintains that the nerve-fibres arise as prolongations of 

 the ganglionic cells located in the retina, and that they enter into union with 

 the central nervous apparatus only secondarily. He is strengthened in his 

 opinion by the conditions in Petromyzon, which he declares to be one of the 

 most valuable objects for the solution of the controversy concerning the origin 

 of the optic nerve. I refer, moreover, in connection with this controversy, to 

 the section which treats of the development of the peripheral nervous system 

 (p. 452). 



(e) The Development of the Accessory Apparatus of the Eye. 



There are associated with the eyeball auxiliary apparatus, which 

 serve in different ways for the protection of the cornea : the eyelids 

 with the Meibomian glands and the eyelashes, the lachrymal glands 

 and the lachrymal ducts. 



The eyelids, the upper and under, are developed at an early period 

 by the formation, at some distance from the margin of the cornea, of 

 two folds of the skin, which protrude beyond the surface. The folds 

 grow over the cornea from above and below until their edges meet 

 and thus produce in front of the eyeball the conjunctival sac, which 

 opens out through the fissure between the lids. The sac derives its 

 name from the fact that the innermost layer of the lid-fold, which is 

 reflected on to the anterior surface of the eyeball at the fornix con- 

 junctivas, is of the nature of a mucous membrane, and is designated 

 as the conjunctiva, or connecting membrane, of the eye. 



In many Mammals and likewise in Man there is during embryonic 

 life a temporary closure of the conjunctival sac. The edges of the lids 



