108 THE THIRD DAY. [CHAP. 



each consisting of a single layer of elongated columnar 

 cells. 



In the subsequent growth of the lens, the development of 

 the hind wall is of a precisely opposite character to that of 

 the front wall. The hind wall becomes much thicker, and 

 tends to oblitei'ate the central cavity by becoming convex on 

 its front surface. At the same time its cells, still remaining 

 as a single layer, become elongated and fibre-like. The 

 front wall on the contrary becomes thinner and thinner and 

 its cells more and more flattened and pavement-like. 



These modes of growth continue until at the end of the 

 fourth day, as shewn in Fig. 31, the central hind wall I is in 

 absolute contact with the front wall el and the cavity thus 

 becomes entirely obliterated. The cells of the hind wall have 

 by this time become veritable fibres which when seen in 

 section appear to be arranged neai-ly parallel to the optic 

 axis, their nuclei nl being seen in a row along their middle. 

 The front wall, somewhat thickened at either side where 

 it becomes continuous with the hind wall, is now a single 

 layer of flattened cells separating the hind wall of the lens, 

 or as we may now say the lens itself, from the front limb of 

 the lens-capsule ; of this it becomes the epithelium. 



The subsequent changes undergone consist chiefly in 

 the continued elongation and multiplication of the lens-fibres, 

 with the partial disappearance of their nuclei. 



During their multiplication they become arranged in the 

 manner so characteristic of the adult lens. 



The lens-capsule mui its adjuncts. In spite of the 

 numerous investigations which have been made in reference 

 to the development of the lens-capsule, its precise mode 

 of origin can hardly even yet be said to be certainly known. 

 Remak was led from analogy to regard it as a product of the 

 mesoblast, though he did not succeed in satisfactorily demon- 

 strating the fact. Kolliker looked upon it as a cuticular 

 membrane thrown off by the superficial cells of the lens, and 

 his view has been very generally adopted. 



Lieberkiihn has given a different account of its origin. 

 According to him the involution of the lens, as we have already 

 stated, carries inwards with it a very thin layer of meso- 

 blast. This remains continuous with the mesoblast surround- 

 ing the eyeball, so that when subsequently the mesoblast 



