ii6 THE INTEGUMF.NTAL SKELETON OF THE IMAGO. 



relations of these parts and of the segmental region of the 

 head. 



The figures are diagrammatic, and only intended to give 

 the results of a long series of observations, made partly on 

 embryos and partly on various adult and larval insects. The 

 head capsule is represented in its embryonic condition, and the 

 nerve centres in the form they assume in the adult of the most 

 highly differentiated Insecta. 



I regard the anterior cephalocele {a cc) as the vesicle of the 

 olfactory lobes (/), the posterior cephalocele (p cc) as the 

 vesicle of the cerebral hemispheres (-7) and their median 

 ventricle. The epicephalic region (ep c) is probably more 

 complex, and corresponds with the optic commissure, the 

 corpora fungiformia {■!), and the dorsal arches of the first 

 three ventral neuromeres (i., ii., m.), whilst the great optic 

 lobes (6') apparently belong to the paracephala (pa c). The 

 latter frequently overlap the segmental parts of the head 

 capsule, especially in the Orthoptera, thus bearing some resem- 

 blance to the opercula in fishes. 



In the Fly-nymph the median parts of the head capsule lie 

 in a deep cleft between the two paracephala, and in close 

 proximity to the ganglia with which they correspond, so that 

 the head appears to be open in the middle line ; but sections 

 show that this appearance is due to the deep infolding of the 

 inner edges of the paracephala. 



I shall hereafter show that the paired lateral invaginations 

 of the epiblast, from which it is stated the pre-oral centres are 

 developed in some arthropods, are sensory vesicles — a view 

 which is borne out by the development of the compound 

 eyes, thus establishing another relation between the arthropod 

 and the vertebrate. 



The nature and relations, as well as the developmental 

 history, of the nerve centres and sense organs will be fully dis- 

 cussed in the sections devoted to the nervous system. 



