."•"• '^'"' -j-cX. 



''''•■•■■■^^^^ft^--ue 



■ u--. 



•on. 



;cor ret 



^ ^-'-'--^ .'prl :c.l. ^ ■'^- 



Text-figure 3. Developineut of tlie anterior median eye. (Diagrammatic.) 



A. Condition of the eye at an early stage in the development of the 

 embryo. 



a. 0. v., anterior optic vesicle. The lumen of the vesicle, lu., is still con- 

 spicuous and opens to the exterior as the semi-circular (crescentic) groove. 

 0. pi., the optic plate, c. f., edge of the ceplialic fold. The ectoderm form- 

 ing the "hood" connects the optic plate with the anterior lip of the optic 

 invagination, c. I., cerebral lobes. 



B. At a more advanced stage. 



ret., the retina, formed by the involuted optic plate, cor., corneal layer, 

 formed by the ectoderm lying above the retinal portion of the eye. o. n., 

 the optic nerve (rudiment) formed by the line of ectodermal cells which 

 connect the retina with the lip of the anterior optic vesicle, v. e., vesicle 

 of the median eye. The term vesicle is, at this stage, purely nominal, 

 since the limien of the sac formed by the involution of the optic elements 

 is contiguous with the lumen of the anterior vesicle. 



C The eye at a considerably advanced stage. 



The post-retinal layer, j)r. L, is formed by certain of the cells which 

 belong to the layer uniting the retina with the anterior optic vesicle becom- 

 ing connected with the cerebral lobe. By the increased growth of the lobe 

 these cells are tin-ned outward until they come to lie directly beneath the 

 retina, thus forming a third optic layer. The sac formed by the involution of 

 the optic elements is now separated from the lumen of the anterior optic 

 vesicle, which becomes practically obliterated. 



That portion of the ectodermal layer which remains attached to the ante- 

 rior optic vesicle, connecting it with the retinal portion of the eye, o. n. 

 in C and D, becomes the optic nerve. 



D. A still later stage. 



The three layers of the anterior median eye l)econie more closely approxi- 

 mated as its development continues. 



The lettering for C and D is the same as in the preceding figures. 



