No. I.] DEVELOPMENT OF THE COMPOUND EYE. ci 



beneath the anterior and outer part of the optic disc, before 

 any striking changes are visible in the external appearance of 

 the embryo. The relations are shown in Fig. 3. The cavity of 

 invagination is small, but comparatively wide, and it retains its 

 size until the lips begin to grow together, when it becomes more 

 flattened. The posterior part of the optic lobe, together with 

 the cord of small cells reaching back to the rudiments of the 

 embryo (Fig. i), give rise to the brain, and need not be followed 

 further here ; the anterior part of the lobes and the invaginated 

 pit alone concern us. The separation of the pit from the parent 

 epiblast is completed at about the time of the budding of the 

 first pair of appendages, and the appearance of the stomodeum. 

 At this time the relation of the invaginated sac to the surround- 

 ing parts is shown in Fig. 4. 



We have now three layers to deal with, all of which are con- 

 cerned in the development of the optic apparatus. The external 

 one is the epiblast or ectoderm {e), while the two others are 

 derived from the invaginated portion of the same primitive 

 layer. The inner of these, the layer which lies against the 

 yolk of the egg, gives rise, as will be seen later, to the chain of 

 ganglia and nerves which lies within the stalk of the adult eye, 

 connecting the optic apparatus with the brain. Hence the 

 name gangliogen (g) may be appropriate for it. The other 

 wall of the optic cavity becomes closely pressed against the 

 ectoderm, and from the fact that it subsequently develops all 

 the retinal portions of the eye, I have called it the retmogen (r). 

 At the stage figured one may notice a correspondence in num- 

 ber, size, and position of the nuclei of all three layers. This 

 equality lasts but a comparatively short time. The optic cavity 

 soon becomes flattened, so that the walls nearly touch, but 

 absolute contact is never reached. At the same time the nuclei 

 of both gangliogen and retinogen are undergoing division, so 

 that the extent of both layers is increased, without, however, 

 losing their primitive character of being one cell in depth. The 

 epidermis does not divide as rapidly, and hence there results a 

 lack of agreement between the nuclei of the retinogen and those 

 of the overlying layer. This is shown in Fig. 5, which repre- 

 sents the eye at the period when two pairs of appendages are 

 outlined. 



There is another feature in this figure which needs a moment's 



