THE ORGANS OF THE OUTER GERM-LAYER. 



473 



the vesicle has now become a solid structure, which, as the lens-core, 

 furnishes the foundation of the lens of the adult. 



The further increase in the size of the lens is an appositional growth. 

 Around the core first formed arise new lens-fibres, which are arranged 

 parallel to the surface of the organ and are united into coats. These 

 lie in layers one over another, which in macerated lenses may be 

 detached like the coats of an onion. All fibres (fig. 268 If, If") 

 extend from the anterior to the posterior surface, where their ends 

 meet one another along regular lines, which in the embryo and the 

 new-born animal have the form of two three-rayed figures, the 

 so-called stars of the lens (fig. 268 vst and list). These exhibit the 

 peculiarity that the rays of 

 the anterior face alternate 

 with those of the posterior 

 face, so that the three rays 

 of one star halve the spaces 

 between the three rays of the 

 other. 



In the adult the figure 

 becomes more complicated, 

 because lateral rays arise on 



each of the three chief rays. 



How have the newly de- 

 posited fibres been formed ? 

 Their origin is ultimately to 

 be referred to the lens-epi- 

 thelium of the front surface 

 of the organ. In these cells 

 figures of nuclear division can 



frequently be observed even in late stages of development. The cells 

 which result from division serve to replace those which grow out 

 into lens-fibres, and are placed upon the already formed layers. 

 The new formation takes place only at the equator of the lens 

 (fig. 267) in the zone of transition (I'} previously described, where, 

 in the adult as well as in the recently born animal, the cubical 

 epithelial cells gradually merge into cylindrical and fibrous elements, 

 as one can convince himself from any properly directed section. 



In the adult, as is well known, there exist no special provisions for 

 the nutrition of the lens, which, after attaining full size, is not much 

 altered, and certainly undergoes only a slight metastasis. With the 

 embryo it is otherwise. Here a more active growth necessitates a 



Fig. 268. Diagram of the arrangement of the 

 lens-fibres. 



One sees the opposite positions of the anterior (vsl) 

 and the posterior (/<*<) stars of the lens. 

 if, Course of the lens-fibres on the anterior 

 surface of the lens and their termination at 

 the anterior star of the lens ; If", continuation 

 of the same fibres Co the posterior star on the 

 posterior surface. 



