800 SUMMARY OF CURRENT RESEARCHES RELATING TO 



halves approach one another above the oesophagus, and gradually take 

 up a dorsal position. 



Two enteric canals are to be found, one in the larva before it 

 comes into the outer world, and the other a week after extrusion ; 

 between these there are morphological as well as histological marks of 

 difference ; the reason for these is to be found in the relations of the 

 intestine to the yolk. It is very interesting to observe that, after 

 the loss of the altered yolk from the crop that organ becomes filled 

 with air, which has certainly a respiratory significance. This fact 

 points to the possibility of a very interesting comparison of the crop 

 with the lung of vertebrates, a comparison which its position makes 

 still more plausible ; but it is one which requires further and com- 

 parative investigation. 



Optic Ganglion of Aeschna.* — M. H.Viallanes finds that the optic 

 ganglion of Aeschna maculatissima is composed of the layer of post- 

 retinal fibres, the ganglionic layer, the external chiasma, the external 

 medullary mass, and the internal chiasma and medullary mass. From 

 each simple eye there is given off a nerve-fibre which, after having 

 pierced the limiting membrane of the compound eye, passes inwards 

 to the ganglionic layer ; this last is a sort of nerve-screen interposed 

 on the course of the post-retinal fibres ; in a well-advanced larva it 

 appears to be protected by two limiting membranes and to be com- 

 posed of three layers. The external layer is formed of unipolar nerve- 

 cells, the prolongations of which pass to the median layer ; this is 

 composed of dotted substance, and has no nuclei in its interior ; the 

 inner layer is formed of dotted substance also, but contains a number 

 of nuclei. During the development of the larva changes are effected 

 in the constitution of the ganglionic layer. The fibres that pass out 

 from it cross completely to form the external chiasma ; this last and 

 the layer of post-retinal fibres undergo during larval life modifications 

 which are merely the result of a movement of translation effected by 

 the ganglionic layer in the course of its development. In the young 

 larva the layer is folded on itself, is at some distance from the eye 

 and very near the brain ; as the insect developes the layer unfolds 

 and comes almost into contact with the eye ; from this there results 

 an elongation of the fibres of the chiasma, and a considerable shortening 

 of the post-retinal fibres. 



The external medullary mass has the form of a groove, which is 

 strongly depressed from before backwards, and flattened from above 

 downwards ; it is completely formed of dotted substance. Of the 

 ganglionic centres connected with the external medullary mass, two 

 are formed of small unipolar cells ; the anterior ganglionic mass con- 

 sists of large unipolar cells, and the internal is formed of an aggrega- 

 tion of nerve-cells which invest the concave surface of the external 

 mass. 



The posterior capsule is directly connected with the external 

 medullary mass by two large bundles of fibres, which are completely 

 independent of the chiasma, and do not intercross. The internal 



* Ann. Sci. Nat.— Zool., xviii. (1884) Art. 4, 34 pp. (3 pis,). 



