SELECTED NOTES. 175 



Head of Blow-Fly (three transverse sections). — From the 

 amount of loose embryonic cells seen in these sections, it is evi- 

 dent that the fly had biit recently emerged from the pupa case. 



On the first of these slides are numerous portions of the brain 

 and optic tract, and as they are fairly thin (about 1/80 mm.) most 

 of the recent discoveries may be compared with them. M. N. 

 Newton states in the Magazine of Natural History^ 1879, P- 397 '- 

 " In the cerebroid or supra-oesophageal ganglia are situated the 

 organs of perception, of memory, of intelligence, etc. Hence 

 they have a more complicated histological structure than the sub- 

 cesophageal ganglia which principally govern the appendages of 

 the mouth. These nerve-centres are nevertheless constructed on 

 the same general plan as the other ganglia. In the middle they 

 present bundles of nerve-fibres, while the nerve-cells principally 

 occupy the periphery." 



In these sections nerve-fibres may be traced from the centre of 

 the oesophageal ganglion to well-defined peripheral nerve-cells. 

 There is here a Hkeness to the Vertebrata, though in almost every 

 other respect, with the exception of the muscles and nerves, we 

 find the opposite. 



The compound eye and optic nerve has been so well worked 

 out by Hickson, that I would refer our friends to his paper which 

 appeared in The Quarterly Journal of Microscopy, No. XCVIII. 

 In Fig. 2 I give a tracing from this paper, showing one facette of 

 the eye and with it terminal fibres, etc. 



The second slide shows several sections through the frontal sac. 

 Lowne believed this to be an olfactory organ, adapted to the 

 appreciation of powerful odours. If we look at the head of the 

 insect as an almost closed sac, bounded by rigid walls, and with 

 all otherwise unoccupied spaces filled with a circulating fluid (the 

 blood), whose communication with the thorax is by a very small 

 neck, and that small space taken up by the oesophagus, nerve- 

 cords, and tracheae, it is evident that the blood could not pass so 

 quickly into or out of the head as would admit of the quick pro- 

 trusion of the proboscis (see Fig. 3). Hence, in the frontal sac 

 there is a beautifully simple contrivance well adapted for such a 

 purpose. It is a simple sac suspended near the upper wall of the 

 head, with the wider surface hanging in numerous folds. The sac 



