24 WILLIAM PATTEN. 



apparently neither connected with surface sense organs nor 

 themselves in a position to receive stimuli from without. 



The median olfactory nerve is of an entirely different 

 nature from the one just described. If differs greatly in size 

 and complexity in different individuals, and if the supposition 

 shortly to be advanced is right, it is better developed in the 

 males than in the females. It arises long after the lateral 

 nerves (after the third larval moult) as an outgrowth from the 

 anterior wall of the median eye tube (PI. 3, fig. 43). In the 

 adult it is a solid nerve composed of two portions, a distal and a 

 proximal one. The latter is composed of a mixture of nerve- 

 fibres and ganglion-cells. The nerve-fibres are not the 

 apparently hollow nerve-tubes seen in the lateral olfactory 

 nerves and elsewhere, but appear to be more solid and refrac- 

 tive, with a yellowish cast, and nuclei here and there. At 

 intervals throughout the proximal portion of the nerve there 

 are spindle-shaped ganglia composed of small densely crowded 

 and deeply stained nuclei, exactly like those in the fore-brain. 

 They vary in number and size, and may extend directly into 

 the brain-tissue at one end of the nerve, or up to the olfactory 

 organ at the other (fig. 18). 



The distal end of the median nerve divides into many 

 diverging branches, which can be followed by means of a hand 

 lens to the posterior edge of the olfactory organ ; they there 

 begin to anastomose, and form a dense plexus underlying the 

 olfactory region, but a considerable number of fibres extend 

 beyond the olfactory region to the neighbouring ectoderm. 

 Here and there the larger strands of the plexus contain small 

 groups of the dark-coloured nuclei, similar to those in the 

 brain ; or the smaller strands may contain a single large 

 tripolar ganglion-cell, with granular protoplasm and a large 

 clear nucleus (fig. 9, g, c). 



A large blood-vessel accompanies the median nerve ; under 

 the olfactory organ it breaks up into numerous branches, some 

 of which are crammed full of blood-corpuscles that stain 

 dark red, and under a low power might be mistaken for 

 ganglia (fig. 18, h. v.). 



