DEVELOPMENT OF THE FLY'S HEAD 



677 



appendage and the pharynx so completely unite that the two 

 soon form a single sac, the head-sac or vesicle (Fig. 629, ft). The 

 walls of this head-vesicle are the later head-wall, the most impor- 

 tant parts of which can now be recognized (the antennae, eyes, 

 rudiments of the beak). It is now necessary that the head-vesicle 

 (Fig. 629, -f , +) be, by the eversion of the pharynx, turned out- 

 ward in order that the head of the pupa may be completed. By 

 this eversion of invaginated parts, the former mouth-opening of 

 the pharynx becomes a neck-section (Fig. 629, +, +) by which 

 head and thorax are now united. (Korschelt and Heider.) 



FIG. 629. Diagram of the changes to pupa of Musca before imago appears; the wing-germs 

 not drawn : as, eye-buds ; at, antennal germs ; & 1 -?' 3 , leg-germs ; bg, ventral nerve-cord ; ff, brain ; 

 k, head-vesicle (originating from the union of the pharynx with the hypophysis, ffirnanhdngen) ; 

 oe, oesophagus; r, germ of the proboscis; **, germ of the forehead; /, //, III, 1st, 2d, and 3d 

 thoracic segments. Based on Kowalevsky and Van Bees, with changes, after Korschelt and 

 Heider. 



The cause of the eversion of the head-vesicle, which Weismann directly ob- 

 served, appears to be due to an increase of the inner pressure through a contrac- 

 tion of the hinder parts of the body. The anterior end of the oesophagus now 

 becomes turned down ventrally corresponding to the conformation of the head 

 of the imago. 



It has been shown that the so-called pharynx is only an invaginated part of 

 the outer surface of the larval head. The brain-appendage Korschelt and 

 Heider consider to be the diverticulum of this irivagination, in which the single 

 parts of the body lie in an invaginated state. They may throughout be compared 

 to the rudiments of the thoracic limbs. All these imaginal buds have been 

 traced back to the invaginated parts of the outer surface of the body, i.e. the 

 ectoderm. 



