106 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



layer lying next to the optic lobe (Fig. 48). Viallanes (1891) has shown 

 that in Mantis the optic lobe {lob 1) frees itself entirely from the eye disk 

 (Fig. 49), the postretinal fibers (/6) later growing out centrifugally from 

 the lobe to join the disk. The same condition was found in Xiphidium 

 by Wheeler (1893) and in Locusta bj^ Roonwal (1937). On the other 

 hand, that the optic lobe in some cases is only partially separated from 

 the eye disk, a strand of nerve tissue which later forms the postretinal 

 fibers remaining between them, has been demonstrated in Forficula by 

 Heymons (1895a) and in Eutermes by Strindberg (1913). In Locusta 



Fig. 49. — Mantis religiosa. Cross section of head through developing eye of an old 

 embryo, (deui) Deutocerebral lobe, (eye) Eye plate (disk), (fb) Postretinal fibers. 

 (ggi- p) External ganglionic plate, (lob 3) Third protocerebral lobe, (stom) Stomodaeuni. 

 (From Viallanes.) 



the nuclei of the optic ganglion near its dorsal edge elongate and send out 

 nerve fibers which go to the retinulae of the eye disk forming the post- 

 retinal fibers. Roonwal found no evidence of fibers being sent out from 

 the retinulae. In the Orthoptera, Dermaptera, and Eutermes the optic 

 lobe is formed by delamination ; in the Coleoptera and Hymenoptera, by 

 invagination. In the latter class the optic disk, or plate, is formed from 

 ectoderm lying outside and immediate!}^ surrounding that destined to 

 form the optic lobes; i.e., the optic lobe and the optic plate develop from 

 separate areas of the ectoderm, whereas in the first class they are formed 

 from the same area. 



The origin of the fibers that constitute the commissures of the brain 

 appears to differ among insects. Those in the commissures of the proto- 

 cerebral lobes are said to arise from the lobes in Locusta, Mantis, Xiphid- 



