369 



part of the antenna, irresp'ective of the action of the other 

 set of muscles. 



The muscles begin to develop in the pupa of four hours; 

 the actual histogenesis of these muscles, which does not differ 

 from that of other muscles, will be described later. 



By this process, then, there is formed the antenna of 

 the imago, a structure which is to be regarded simply as a 

 highly sensitive portion of the integument,' modified and 

 grown out in such a way as to permit of a maximum of 

 efficiency in the action of the sense cells, which it bears. 



The Or gams of Vision. 



The Corrvpoiond Eye. — But of all the changes undergone 

 by the ectoderm as it gradually develops in the larva and 

 the pupa, the most remarkable are those which take place at 

 the sides, and in front, of the head. Here the ectodermal 

 cells become exceedingly specialized, and, while retaining 

 their primitive function of acting as a protection for the 

 internal organs, as well as, to a certain extent, their capacity 

 for secreting a cuticle, yet become modified, and disposed 

 in such a manner that the terminations of outgrowing nerve 

 fibres from the brain, which come to end in close relation with 

 them, may become stimulated in certain ways by the light 

 rays emitted from external objects, the vague impressions of 

 which become modified, as a result of their physical media- 

 tion, into what must now be very highly specialized sensations. 



As a result of the processes, which begin in the embryo, 

 and are continued right throughout larval and pupal life, the 

 great compound eyes and the three smaller ocelli become 

 developed. 



The formation of the compound eyes will be considered 

 first. Already in the larva of the first instar the ectoderm 

 of the head, on either side of the brain, has begun its modi- 

 fied course of development. The ectoderm at this stage con- 

 sists of a large number of cells disposed roughly in three layers 

 (fig. 55). Although no examination of eyes in unhatched 

 embryos was made, yet there can be no doubt that the cells 

 of these layers are formed as a result of a division of vertically 

 elongated cells, the disposition of the cells at this stage being 

 such as to indicate that they had arisen from those of the 

 middle layer. Several individual cells are shown isolated in 

 fig. 54 for greater clearness. The cells of these three layers 

 can already be distinguished morphologically. Those of the 

 external layer are rather short and generally conical in shape; 

 the middle layer cells are long and generally spindle-shaped, 

 with the nucleus in their middle, while the cells of the inner 



