v.] THE CHOROIDAL FISSURE. 101 



level of the stalk, some such figure would be gained as that 

 shewn in Fig. 30 E. Here the fissure / is obvious, and the 

 communication of the cavity vh of the secondary vesicle with 

 the outside of the eye evident; the section of course would 

 not go through the superficial epiblast. Lastly, a section, 

 taken perpendicular to the plane of the paper along the line 

 z, i.e. through the fissure itself, would present the ap- 

 pearances of Fig. 30 F, where the wall of the vesicle is 

 entirely wanting iu the region of the fissure marked by the 

 position of the letter/. 



The fissure such as we have described it exists for a short 

 time only. Its lips come into contact, and unite (in the 

 neighbourhood of the lens, directly, but in the neighbourhood 

 of the stalk, by the intervention of a structure which we 

 shall describe presently), and thus the cup-like cavity of the 

 secondary optic vesicle is furnished with a complete wall 

 all round. The interior of the cavity is filled by the 

 vitreous humour, a clear fluid in which are a few scat- 

 tered cells. 



In the foregoing account of the formation of the secondary optic vesicle, 

 and of the fissure, as the results of a process of unequal growth, we have fol- 

 lowed the account of Lieberkiihn (Uber das Auge des Wirbelthierembryos, 

 Schriften der Gesedschaft zur Belorderung der gesammten Naturwissenschatten 

 zu Marburg. Bd. lo. 1S72). Their origin is more generally described as 

 being due to a doubling up of the primary vesicle from the side along the 

 line of the fissure at the same time that the lens is being thrust in in front. In 

 mammalia, tlie doubling up is said to involve the optic stalk, which becomes 

 flattened (whereby its original cavity ia obliterated) and then folded in on itself, 

 so as to embrace a new central cavity continuous with the cavity of the 

 vitreous humour. 



According to Lieberkiihn the optic stalk in birds is never so folded up, 

 but is converted into the optic n<irve by the gradual obliteration of its primary 

 central cavity through increased thickening of the walls. The optic nerve 

 of the bird, moreover, contains no arteria centralis retinae, while the involu- 

 tion of the optic stalk into the optic nerve was supposed to have for its purpose 

 the introduction of a quantity of mesoblast into the interior of nerve, in order 

 to form the artery. 



According to Remak and the majority of observers after him, no mesoblast 

 whatever exists between the external epiblast and the optic vesicle, at the 

 point where the former is thrust inwards to form the lens, and hence this 

 organ carries with it in its involution no mesoblast whatever to serve as a 

 rudiment of either the vitreous humour or the capsule of the lens. They 

 described the vitreous humour as being formed entirely out of the meso- 

 blast which was intruded from the exterior of the eye through the choroidal 

 fissure, and Kolliker considered the capsule of the lens as a sort of cuticular 

 excretion from the surface of the lens itself. LieberliUhn on the other hand 

 states that shortly after the commencement of the involution of the lens there 



