THE SENSORY ORGANS. 



connective tissue-cells between the inner ectoderm-mass, this tissue 

 forming a continuous layer between the two. From this, i.e., from 

 mesodermal elements, the lens, according to PATTEN, is formed, while 

 the inner ectodermal mass yields the principal constituent of the eye. 



FIG. 28. A section through an eye of Pecten (after PATTEN from HATSCHBK'S Text- 

 bonk of /oology). l t cornea ; %, lens ; 3, pigmented ectoderm ; ^, blood- 

 sinus round the lens ; 5, retina, with superficial ganglionic layer and backwardly 

 directed rods ; 6, pigment-layer, with the tapetum lying in front of it ; 7, optic 

 nerve. 



The way in which the various layers, the ganglionic cell-layer, the 

 retina, the argentea, and the tapetum, etc., arise out of this mass 

 is described, but these difficult points are not made sufficiently 

 clear. 



Further details concerning the ontogeny of these very peculiar eyes and 

 especially as to the origin of the rods are much to be desired. The solution 



F 



