60 T. H. MORGAN. 



arrangement of the elements of the middle layer is not well 

 shown in the figure, but other sections show that they are essen- 

 tially as in the adult, and that small rods, bacilli, are developed 

 along the sides of the inner ends. The eye also shows in other 

 sections a distinct raphe. The inner layer is not so much pig- 

 mented as in the adult. The essential interest of this figure lies 

 in the relationship it reveals between the first, the outer, and the 

 second, the middle, layers of the eye. If we follow from above 

 downwards the layer of corneal hypodermis, we find that when 

 it reaches the lower angle of the eye it suddenly turns inwards 

 and becomes directly continuous into the middle layer of the eye, 

 and that the large nuclei of the outer can be traced continuously 

 into the nuclei of the middle layer at this point. 



This figure does not show the relationship between the pig- 

 ment layer and the hypodermis, but others do, and it may be 

 stated that just below the eye the pigment layer becomes con- 

 tinuous into the hypodermis, and at the upper part of the eye, as 

 seen in this figure, the middle and inner layers are in close con- 

 tact. We may interpret this condition of the eye by referring it 

 to an invagination of the ectoderm, the opening, if it exists, 

 being at the lower corner of the middle layer ; that the two 

 walls of the invagination are of different thicknesses, etc., the 

 middle layer being one and the pigment layer the other; and, 

 finally, the whole invagination being pushed over to one side 

 — dorsalwards. I think this interpretation of the conditions is 

 not forced, and is the explanation of the conditions shown not 

 only by this but by other figures. 



Figs. 49 and 50 are from a larva having three pairs of walking 

 legs. (See Fig. XVII, Plate Y.) These two figures belong to 

 the same series, and are neighboring sections of one of the pos- 

 terior eyes. The animal is about ready to moult, so that we find 

 two distinct chitinous coats, the one closely adhering the other 

 over the latter and not directly connected to it. Fig. 49 is some- 

 what to one side of the middle line, especially in the upper part 

 of the eye, but near to it in the lower part. The corneal-hypo- 

 dermal cells at the lower corner of the eye pass into the cells of 

 the middle layer. The outlines of the cells are better seen than 

 in the previous section, and small rod-like bodies are seen at 

 their inner ends. The middle layer is smaller than in the pre- 



