254 



AYERS ON THE DEVELOPMENT 



fibrous portion of the brain, leaving a cavity about the nucleus and its radial fibres. 

 The nervous cord in the hatched embryo embraces 17 pairs of ganglia. Numbering 

 from before backwards, 1, forms the brain ; 2, 3, 4, the suboesophageal ; 5, 6, 7, the thoracic ; 

 8-17, the abdominal ganglia. During embryonic life the brain shows no traces of the spe- 

 cialized parts (e. g. calices, trabeculae, central body, etc.) found in the adult. In pi. 22, 

 fio-. 1, are figured three pairs of large nuclei which occur in the adjacent walls of the suc- 

 cessive thoracic ganglia. PI. 22, fig. 7, represents these nuclei more highly magnified. 

 No nuclear membrane is distinguishable. The nuclear substance appears finely granular in 

 the sections, and near the centre of each of the nuclei occupying the anterior edge of a gan- 

 glion is a bar-shaped nucleolus, while the nuclei lying in the posterior edge of a ganglion 

 possess several small round nucleoli. The significance of these nuclei is unknown. The 

 optic lobes (pi. 20, figs. 22 and 23) are first seen as rounded projections on the outer sur- 

 face of the hind part of each half of the brain. By a gradual growth they appose them- 

 selves to, and finally fuse with, the much thickened ectoderm near the base of the 

 antennae. 



Before the differentiation of the optic lobes of the brain, the ectoderm just posterior to 

 the base of the antennae is raised into a pair of lenticular elevations which ultimately form 

 the ectodermic parts of the eye — i. e. the cornea, lenses, rods, and retina. (PL 23, fig. 16.) 

 The cells in this elevation are at first colored with a brown pigment, but the color disap- 

 pears about the time of the closure of the dorsum. The surface of the elevation becomes 

 papillate by the projection of the cells which form the simple lenses. This condition 

 persists (pi. 20, fig. 47 ; pi. 25, fig. 31) until after the first ecdysis of the hatched insect, 

 when the cornea becomes smooth and glassy. 



The following table is self explaining : — 



A. Ectoderm. — 



Origin. 

 From the superficial cells on the dorsal 

 side of the egg in the future cephalic 

 region. 



B. Mesoderm. — 



C. Endoderm. 



From indifferent yolk cells ; from the 

 inner ends of the cells of the germinal 

 band (?). 



From indifferent yolk cells; from the 

 superficial cell layer — the blastoderm or 

 yolk sac. 



Fate. 

 It becomes the " hypoderm " of the 

 body and its appendages, — the gills, 

 wings, and ventral appendages, — also 

 the nervous system, the tracheae, 

 the epithelial lining of stomodaeum 

 and proctodaeum,the salivary glands 

 and the Malpighian vessels ; and 

 forms by secretion the cuticula of 

 the first three structures. 



It becomes, muscular layers of the 

 enteric tract, sexual organs, heart, 

 segmental muscles, peritoneum, and 

 segmental organs (?). 



It forms the epithelial lining of the 

 mesenteron and furnishes the cor- 

 puscles of the vascular fluid. 



The germs of the sexual organs do not appear until after revolution and the beginning 

 of the formation of the dorsum. (PI. 22, figs. 1, 4, 5). They are first seen as two irregular 

 groups of amoeboid cells, belonging to the splanchnic layer of the mesoderm on either side 

 of the dorsal vessel. Later they assume the form of spherical masses, which soon elongate, 

 becoming first oval, then cylindrical and finally pear-shaped. The ovaries at their ante- 

 rior ends become small and rod-like, and the anterior end of each rod is connected with 



