TISSUES OF THE BODY. 



279 



nucleus may reach 1'194. The index of refraction amounts, in the 

 external strata, to 1-4071 according to Krause; in the middle to 1-4319, 

 and in the central to 1'4564. 



161. 



The lens is developed from a doubling-in of the superficial layer of cells 

 coating the embryonic body or the corneous layer, which has been already 

 discussed in considering the epidermis. 



But even at a very early period it appears as a structure completely sepa- 

 rated from the layer just mentioned. It is hollow in the interior, and 

 has very thick walls, which are bounded by a transparent membrane. 

 These walls are formed of several strata of elongated cells. From them, 

 possibly, the excretion of the homogeneous substance has taken place, 

 which subsequently solidifies into a capsule for the whole organ. In our 



Fig. 269. er-e, Cells from the lens of a 

 foetal pig two inches long, a, original 

 cells; 6, other elongated specimens; 

 c, some more so still, passing into the 

 form of tubes; d, epithelium of the 

 lens of a human embryo at eight 

 months; e, cells from the so-called 

 humor Morgagnii. 



Fig. 270. Fibres from the lens of a human 

 foatus at eight months, a, fibies with 

 one nucleus; 6, another, which still 

 shows its cellular character; c, flattened 

 form, as seen from the side; d, fibres 

 with two or three nuclei. 



opinion, however, the capsule is a modified deposit from the adjacent 

 connective-tissue. These cells gradually fill up, it is supposed, with their 

 descendants, the central cavity, and become developed in most cases into 

 lens tubes or fibres, while a certain remainder only, preserving their 

 original characters, constitute the epithelium of the capsule seen on its 

 anterior internal surface. 



In young embryos we have an opportunity of studying the fibres of 

 the lens in process of development (fig. 269, a-e). 



In more advanced foetuses, as for instance in the human towards the 

 19 



