408 DEVELOPMENT OF ELASMOBRANCH FISHES. 



The lens arises in the usual vertebrate fashion. The epiblast 

 in front of the optic vesicle becomes very much thickened, and 

 then involuted as a shallow pit, which eventually deepens and 

 narrows. The walls of the pit are soon constricted off as a nearly 

 spherical mass of cells enclosing a very small central cavity, in 

 some cases indeed so small as to be barely recognizable <P\. 15, 

 fig- 7> 0- The pushing in of the anterior wall of the optic vesicle 

 towards the posterior takes place in quite the normal manner ; 

 but, as has been already noticed by Gotte 1 and others, is not a 

 simple mechanical result of the formation of the lens, as is shewn 

 by the fact that the vesicle assumes a flattened form even before 

 the appearance of the lens. The whole exterior of the optic- 

 cup becomes invested by mesoblast, but no mesoblastic cells grow 

 in between tJie lens and tJie adjoining wall of the optic cup. 



Round the exterior of the lens, and around the exterior and 

 interior of the optic cup, there appear membrane-like structures, 

 similar to those already described round the spinal cord and 

 other organs. These membrane-like structures appear with a 

 varying distinctness, but at the close of stage K stand out with 

 such remarkable clearness as to leave no doubt that they are 

 not artificial products (PI. 15, fig. I3<?) 2 . They form the rudi- 

 ments of the hyaloid membrane and lens capsule. Similar, 

 though less well marked membranes, may often be seen lining 

 the central cavity of the lens and the space between the two 

 walls of the optic cup. The optic cup is at first very shallow, 

 but owing to the rapid growth of the free edge of its walls soon 

 becomes fairly deep. The growth extends to the whole circum- 

 ference of the walls except the point of entrance of the optic 

 nerve (PL 15, fig. 13*7), where no growth takes place; here accord- 

 ingly a gap is left in the walls which forms the \vell-known 

 choroid slit. While this double walled cup is increasing- in size, 

 the wall lining the cavity of the cup becomes thick, and the 

 outer wall very thin (fig. 13^). No further differentiations arise 

 before the close of stage K. 



The lens is carried outwards with the growth of the optic 

 cup, leaving the cavity of the cup quite empty. It also grows in 

 size, and its central cavity becomes larger. Still later its anterior 



1 Ent-wickhingsgeschichte d. Unke. 



' The engraver has not been very successful in rendering these mernbrane^. 



