THE VITREOUS HUMOUR. 87 



afterwards disappear. The thin anterior layer remains throughout life as a simple 

 layer of cubical cells, and forms the so-called lens-epithelium; but the cells of the 

 posterior layer grow forwards into the cavity of the lens-vesicle as the lens-fibres : 

 those in the middle being the longest and straight, while the rest are slightly curved 

 with their concavity towards the equator, and become gradually shorter towards the 

 circumference, where they pass through gradually shortening columnar cells (transi- 

 tional zone) into continuity with the anterior epithelium. By the growth of these 

 fibres the cavity of the lens- vesicle becomes obliterated. 



In this manner the central part of the lens is formed, and it consists in the main 

 of fibres which pass in an antero-posterior direction. The remainder of the lens is 

 formed of fibres which are so disposed as to curve round its margin and over the ends of 

 the first formed fibres ; they are, moreover, deposited in successive layers and in 

 three (or more) separate sections, so that their ends abut against one another in front 

 and behind along tri-radiate (or multi-radiate) lines, such as may be seen in the 

 macerated lens. These later deposited fibres are all formed at the equator (at the 

 transitional zone), where chiefly cell-multiplication takes place, and they grow hence 

 meridionally backwards over the ends of the already developed antero-posteriorly dis- 

 posed fibres of the central part of the lens. 



The capsule of the lens is early visible as a thin homogeneous membrane, the 

 origin of which is still undetermined. According to some observers (Lieberkiihn, 

 Arnold, Lowe) it is derived from a thin layer of mesoblast, which passes in between 

 the lens and the optic cup ; according to others (Kolliker, Kessler, Balfour), it 

 appears before any mesoblast has passed in, and they therefore regard it as a 

 cuticular deposit from the lens cells. In the human embryo, His figures mesoblast 

 as existing from the first between the lens invagination and the optic cup (v. fig. 97). 



In connection with, this question it must be remembered that the substantia propria of the 

 cornea (see below), which is formed of connective tissue, and is therefore mesoblastic in 

 nature, also at first makes its appearance as a homogeneous deposit before any mesoblast cells 

 have passed in behind the corneal epithelium. 1 Its chemical nature, and its continuity at 

 the equator with the suspensory ligament and hyaloid membrane, certainly point to the lens 

 capsule as being a connective tissue, i.e. a mesoblastic structure. 



Although the fcetal lens like that of the adult is itself non- vascular, it is nevertheless 

 externally freely supplied with blood-capillaries, which form a vascular tunic completely 

 surrounding it outside the capsule. These capillaries are supplied by a branch of the arteria 

 centralis retince which passes forwards through the centre of the vitreous humour ; in 

 front, at the margin of the pupil, they come into continuity with the vessels of the iris. The 

 most anterior part of this vascular tunic forms a membrane which closes the aperture of the 

 pupil in the middle peiiods of fcetal life. In the human eye the whole tunic, together with 

 the artery which supplies its vessels, becomes atrophied and is lost sight of before birth, but 

 in some animals faz pupillary membrane remains apparent for a few days after birth. 



The vitreous humour appears to be formed from the mesoblastic tissue which 

 has passed in between the lens and the inner layer of the optic cup by a gradual 

 formation of a large quantity of ground-substance, whilst the cells of the tissue 

 almost entirely disappear. The development of the hyaloid membrane has not been 

 fully traced out, and the same may be said with regard to the zonule of Zinn. They 

 are probably both formed by part of the same mesoblast as forms the vitreous humour 

 (Lieberkiihn, Angelucci). 



The corneo- sclerotic coat, the choroid coat, and the iris are all derived from tho 

 mesoblast surrounding the optic cup. 



The corneal epithelium is a portion of the external epiblast, which originally 

 rests against the front of the lens rudiment. The substantia propria cornea 

 first appears in the chick as a thin homogeneous layer lying immediately within 



1 Kessler. however, looks upon this homogeneous deposit as being also a cuticular deposit formed by 

 the epithelial cells. 



