764 



DEVELOPMENT OF THE ORGANS OF SENSE. 



time as a broad white line, particularly obvious in the embryo bird, 

 running from the circumference in upon the lens. 



The lens is developed in the part of the cuticle opposite to the -most 

 projecting part of the primary optic vesicle, or at the place where 

 this vesicle comes in contact with the surftice of the head. In this 

 situation there is seen from a very early period a thickening of the 

 epiblast, which seems to reside chiefly in its deeper layer of cells, and 

 in birds and mammals it would appear that an actual involution 

 of the cuticle takes place, so that first an open follicle and next an 

 enclosed ball of cuticle is formed. Although, however, both the 

 corneous and the deeper layer (sensory of Strieker) of the cuticle are 

 enclosed, it is only the cells of the deeper layer which undergo de- 

 velopment into the fibres of the lens. The ball of the lens separating 

 from the external cuticle, which passes freely over the surface, a cavity 

 filled with loose cells, the remains of cells of the corneous layer, exists 

 for a time within the lens ball. Then the cells of the hinder or inner 

 wall are seen to rise from the bottom by their elongation, and thus 

 a rapid growth of fibres from that side of the ball takes place, while 

 the anterior or outer wall undergoes no similar change, but retains its 

 simply cellular structure. Figures 565, and 566, show sufficiently 

 clearly the manner in which the fibres thus developed from cells rise 

 fi'om the bottom of the lens ball and come to constitute the solid part 

 of the lens. 



The optic cup receives the enlarging lens in its anterior and lower 

 opening, and the reflected margins of the cup closely embrace the 

 margin of the lens ; but there is a considerable space intervening 

 between the lens and the hollow of the optic cup (or secondary ocular 

 vesicle), which comes to be occupied by the vitreous humour. Into this 

 space connective tissue and blood-vessels developed from mesoblastic 



Fig. 565. — Sectiojt of the Eye in an Embryo-C^lf 

 OF 18 MM. LONG, MAGNIFIED (from JuUus Amold). 



The posterior or inner v.-all of the lens follicle is now 

 much thickened by the elongation of its fibres, each 

 of which presents a nucleus, and the whole causing a 

 bulging of the posterior wall. The outer la^'er of the 

 lens capsule consists of columnar cells. The cavity of 

 the lens follicle is still visiljle, but is now widened 

 and flattened. Two layers of mesoblastic tissue are 

 now visible between the lens and the cuticle, viz., a 

 deeper vascular layer and a superficial non-vascular 

 one containing nuclei. The secondary optic capsule is 

 now occupied bj' connective tissue nuclei and numerous 

 blood-vessels. The retinal section of the primitive 

 ocular vesicle is now thicker. Pigment begins to aja- 

 pear in the choroidal section, and numerous blood- 

 vessels surrovmd the whole exterior. 



elements are projected from below, so as 

 ..'^,|y| to furnish the materials for the formation 



^ '"^r of the vitreous humour and the blood-vessels 



-W which pass through it to the lens, and also 



to surround the lens with vascular and 

 fibrous elements, out of which are produced the capsulo-pupillary mem- 

 brane, and probably also the capsule of the lens. It results from tlie 



