Royal Microscopical Society. 
261 
of the blastoderm. As yet a complete series of investigations are 
wanting, but I have traced the steps of development sufficiently to 
allow me to state that the great procephalic lobes which exist in the 
half-developed embryo become folded inwards, and lie one on either 
side of the alimentary canal during the whole period of larval life, 
and that the nervous system of the larva, by the modification of 
which that of the imago is developed, is formed as a thickening of 
that portion of them which originally bounds the primitive fork at 
the anterior extremity of the ventral groove. These involuted pro- 
cephalic lobes — and they are nothing else — form a portion of the 
imaginal disks of Weissmann, whilst the eyes, antennae, and mouth- 
organs, are ultimately developed from cellular outgrowths at the 
bases of the same structures, just as they are in the Crustacea. 
The nerve centres of the higher insects, such as the blow-fly and 
the higher social Hymenoptera, are even more remarkably like those 
of the Yertebrata, for a pair of cerebroid ganglia are developed in 
them in the most anterior portion of the procephalic lobes. These 
cerebroid ganglia were first pointed out in the Hymenoptera by 
Dujardin,* The ganglia in question are pedunculated convoluted 
nerve centres, situated at the anterior and upper part of the pre- 
stomal nerve centres ; they are united by a well-marked commissure, 
never give off nerves directly, and are further connected with the 
great ganglia of sensation by peduncles, just as the cerebrum is 
in the Yertebrata. 
At present I have not, it is true, succeeded in tracing the develop- 
ment of these cerebroid ganglia, but do not doubt I shall ultimately 
do so. From their position it is not difficult to see that they must 
originate from the anterior (in after-development posterior by flexion) 
portion of the procephalic lobes. 
Having thus pointed out to you very briefly the present condi- 
tion of research as to the relation of the nervous system of the 
Arthropoda and Yertebrata, I will draw your attention to the relation 
of the nervous system of the higher Annulosa to that of the Annelida 
and Annuloida generally. 
The nervous system of the Echinodermata will serve well as a 
starting point. It consists, as you well know, of a band of nerve 
tissue surrounding the pharynx, and composed of nerve fibres with 
nerve cells scattered amongst them. From this circular ganglion 
band radiating ganglion bands (usually five in number), having the 
same structure, pass with the ambulacral vessels towards the antam- 
bulacral, or, as it is sometimes called, dorsal region, that is, towards 
the anal pole of the animal. 
If I may be allowed to speculate on this subject, it seems to me 
probable that the primitive nerve ring is connected with that invo- 
lution of the dorsal integument of the embryo from which the ambu- 
lacral system is developed, and is hence derived from an external 
* ‘ Ann. Sc. Nat.,’ Series iii., T. xviii., p. 231. 
