DEVELOPMENT OF THE FLY'S HEAD 675 



part through a sort of differentiation and separation of the innermost layer 

 of the ectodermal part, and Van Ilees has, in general, confirmed this view. 

 Kowalevsky, on the other hand, is inclined to the view that the mesodermal 

 part of the imaginal bud is derived from the embryonal cells of the mesoderm. 

 He finds scattered throughout the mesoderm, under the hypodermis of the larva, 

 so-called wandering cells (Fig. 032, A, w), which are different in appearance 

 from the leucocytes and from the elements from which the formation of the 

 nifsodermal parts of the imaginal rudiments proceed. Kowalevsky is inclined 

 to believe that there are in each segment rudiments of the imaginal mesoderm, 

 but which are so delicate and indifferent that we cannot find them in the first 

 stages of their origin. From these mesodermal imaginal rudiments the above- 

 mentioned wandering cells of the mesoderm are derived, which afterwards come 

 into connection with the ectodermal portion of the imagiual buds. 



Still more complicated and difficult to understand is the develop- 

 ment of the head-section of muscids. We must remember that in 

 muscid larvae the head-section exists in its most rudimentary form, 

 being the result of extreme modification and degeneration. The 

 small size of the head is also due to the fact tha.t it is more or 

 less retracted within the thoracic region. Then, as shown by the 

 researches of Weismann, in the last embryonal stages, the forehead, 

 mandibles, and the whole region of the head around the mouth 

 invagiiiate and form a sunken cavity (Fig. 628, p), in which the 

 chitiuous supports of the hooks characteristic of muscid larvae are 

 soon developed. This sunken part of the head, at whose inner end 

 is the oesophagus, is called by the not entirely appropriate name of 

 " pharynx," and it must at present be remembered that the hollow 

 space thus named is not a part of the digestive canal. It is an in- 

 vaginated section of the head, and the formation of the head of the 

 imago mainly depends on the evagination of this region. 



The first rudiments of the most important parts of the head (eyes, 

 antennae, and forehead), occur in the youngest larvae as paired masses 

 of cells which lie in the thorax next to the two halves of the brain 

 (for this reason called by Weismann "brain-appendages"), which 

 are from their first origin connected with the pharynx, and may be 

 regarded as the imaginal buds of the head. These appear very 

 soon in later stages in the shape of elongated sacs widening at 

 the hinder end (Fig. 628, A and B, 7i), which from their origin are 

 to be regarded as evaginations of the pharynx. Very soon epithelial 

 thickenings appear in the wall of this sac-shaped brain-appendage, 

 in which the rudiments of the parts of the future head may be 

 recognized. 



Disk-shaped thickenings in the hinder widened part of the brain- 

 appendage form the rudiments of the compound eyes, which there- 

 fore may be called the eye-buds. On the basal surface of the 



