ANATUMY OF THE NERVE CENTRES. 439 
dorsal band. Such fibres, however, may be, and probably 
are, connected with the cortex in other parts of the thoracic 
ganglion. 
The arrangement of the cells and fibres and their relations 
to the nerve roots in the thoracic ganglion are in consonance 
with the view that the ventral chain, which it represents, has 
functions similar to those of the medulla spinalis and medulla 
oblongata in the Vertebrata. I shall hereafter show that the 
evidence we possess indicates that the ventral chain in the 
Arthropoda consists of a series of reflex centres and conducting 
cords. The reflex character of the thoracic ganglia becomes 
very apparent after the cephalo-thoracic cord has been divided 
in the Diptera; an indication that the cephalic centres exhibit 
an inhibitory influence on their reflex functions. 
Morphology of the Thoracic Nerve Centre.—Although sections 
through the thoracic nerve centre exhibit considerable devia- 
tions from the typical form of the ventral ganglionated cord of 
the more generalised Arthropoda, there can be no doubt that 
it is composed of the three pairs of ventral ganglia cor- 
responding to the three thoracic segments, and of two or more 
pairs of abdominal ganglia. The segmentation of the neuro- 
blast only occurs in the early stages of the formation of the 
nymph; there is no such segmentation in the larva. The 
four anterior segments, like those of the Pupipare, become 
far larger than any of the others, which apparently undergo 
absorption or, after having become greatly reduced in size, 
are fused with the fourth segment (compare Figs. 8 and 9, 
Pl. II.). The subsequent development of the dorsal ganglia 
takes place by the enlargement and sub-division into two parts 
of each of the primitive thoracic ganglia (see ‘Development 
of the Nervous System’). 
The union of the abdominal ganglia with the thoracic is by 
no means confined to the Diptera and Hymenoptera, but is 
apparently characteristic of all the higher groups of insects. 
Thus Herold [140] has shown that the first pair of abdominal 
ganglia are fused with the thoracic in the butterfly, Pontia 
Brassice ; whilst the figure given by Straus Durckheim [40] 
