In order to understand how this space is developed, the position of the optic vesicle and the relations of its stalk must be borne in mind.
The vesicle lies at the side of the head, and its stalk is directed downwards, inwards and backwards. The stalk in fact slants away from the vesicle. Hence, when the involution of the lens takes place, the direction in which the front wall of the vesicle is pushed in is not in a line with the axis of the stalk, as for simplicity’s sake has been represented in the diagram ([fig. 285]), but forms an obtuse angle with that axis, after the manner of [fig. 286], where s´ represents the cavity of the stalk leading away from the almost obliterated cavity of the primary vesicle.
[Fig. 286] represents the early stage at which the lens fills the whole cup of the secondary vesicle. The subsequent condition is brought about through the rapid growth of the walls of the cup. This growth however does not take place equally in all parts of the cup. The walls of the cup rise up all round except that point of the circumference of the cup which adjoins the stalk. While elsewhere the walls increase rapidly in height, carrying so to speak the lens with them, at this spot, which in the natural position of the eye is on its under surface, there is no growth: the wall is here imperfect, and a gap is left. Through this gap, which afterwards receives the name of the choroidal fissure, a way is open from the mesoblastic tissue surrounding the optic vesicle and stalk into the interior of the cavity of the cup.
Fig. 286. Diagrammatic section of the eye and the optic nerve at an early stage. (From Lieberkühn.)
To shew the lens l occupying the whole hollow of the optic cup, the inclination of the stalk s to the optic cup, and the continuity of the cavity of the stalk s´ with that of the primary vesicle c; r. anterior, u. posterior wall of the optic cup.
From the manner of its formation the gap or fissure is evidently in a line with the axis of the optic stalk, and in order to be seen must be looked for on the under surface of the optic vesicle. In this position it is readily recognised in the embryo seen as a transparent object ([fig. 118], chs).
Bearing in mind these relations of the gap to the optic stalk, the reader will understand how sections of the optic vesicle at this stage present very different appearances according to the plane in which the sections are taken.
When the head is viewed from underneath as a transparent object the eye presents very much the appearance represented in the diagram ([fig. 287]).
A section of such an eye taken along the line y, perpendicular to the plane of the paper, would give a figure corresponding to that of [fig. 288] D. The lens, the cavity and double walls of the secondary vesicle, the remains of the primary cavity, would all be represented (the superficial epiblast of the head would also be shewn); but there would be nothing seen of either the stalk or the fissure. If on the other hand the section were taken in a plane parallel to the plane of the paper, at some distance above the level of the stalk, some such figure would be obtained as that shewn in [fig. 288] E. Here the fissure f 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 appearances of [fig. 288] F, where the wall of the vesicle is entirely wanting in the region of the fissure marked by the position of the letter f. The external epiblast has been omitted in this figure.