The Eyes of Dactylopius. 83 



l'oiirth stalle, a few large free cells are Seen about the base of the eyes, 

 not only of the ventral eyes but near the dorsal and the primary eyes 

 as well. Thev occur sinfjly or in groups of two to three; are approxi- 

 niately triangulär in outline and of a coarsely granulär coniposition 

 with a good sized nucleus in which there is a clearly defined reticula- 

 tion (Fig. 7, 12, 14). They are evidently migratory cells. What their 

 function can be is not clear, but the fact that they are present where 

 such great niorphological changes are occurring renders it probable 

 that they have a phagocytic function. At this time, too, not long 

 after the nerve has appeared there may be seen in the cerebral ganglia, 

 especially where the nerve enters, several large, oval nuclei about 

 which however, no cell body is evident. Similar nuclei can also be 

 found at various points in the nerve (Fig. 4, 6, 15). They are 

 undoubtedly the nuclei of ganglion cells. 



It was mentioned that the cells bearing the visual rods became 

 pushed below the adjoining hypodermis. Hereupon, after both the 

 Visual Tods and the nerve have advanced somewhat in their development, 

 during the first half of the fourth stage, a change occurs in the hypo- 

 dermis immediately about the eye, preparatory to the formation of 

 the corneal hypodermis. The manner in which this originates is perhaps 

 the most interesting and characteristic process in the entire development 

 of the eye; it is certainly not the common mode of formation for this 

 layer. The condition in the eyes of Acilius as described by Patten (87), 

 conforms most closely to the present case. The layer is composed of 

 cells which have thus far not been included in the optic thickening 

 and is the result of the following changes; an enlargement of the hypo- 

 dermis surrounding the optic thickening; a sinking down of the latter 

 and a subsequent crowding in of the enlarged cells from all sides toward 

 the Center of the eye to form a layer above the visual rods. As a con- 

 sequence the cells of the original thickening become the proximal 

 layer and constitute the retina. 



These changes, as already mentioned, begin about the end of the 

 third stage when the cells of the optic thickening separate from the 

 adjoining hypodermis with which they have up to this time been in 

 contact. These surrounding hypodermal cells then increase greatly 

 in size, their cytoplasm undergoes quite a change in aj^pearence, be- 

 coming coarsely granulär, antl the nuelfi likewise enlarge and become 

 vacuolated. During this process the partial invagination of the retinal 

 portion has been occurring. The future corneal hj^odermis has mean- 

 while crowded up, about the retinal cells and now b.y a centripetal 



6* 



