OM THE BRAIN OF THE COCKROACH. 343 



cal commissures passing from the front to the back of the 

 brain. He mentions also that, in the bee, he has been 

 able to'dissect out the process which passes to the front of 

 the brain, as described by Dujardin. It is this which, when 

 seen from before, gives the appearance of a central nucleus 

 in each hemisphere. The head of the mushroom body- 

 he described as forming a complete ring, which he was . 

 able to separate from the surrounding parts. From what 

 we now know of the structure of these mushroom bodies, it is 

 clear that these parts must have been separated from 

 their attachments before they could give the appearance of 

 closed rings. We shall, I think, see presently that Dujar- 

 din was much more correct in speaking of them as folded 

 discs. 



Ur. Dietl, in 1876 (^ Zeitsch. wissen. Zool.,' Band xxvii, 

 p. 488), published an elaborate description of the brains of 

 the bee, mole-cricket, grasshopper, &c. The method em- 

 ployed by this author was, to cut up in definite directions 

 brains which had been hardened in osmic acid. In the 

 main the results of his observations accord with those of 

 Dujardin and Kabl Riickhard. He agrees with them as to 

 the existence in the bee of two mushroom bodies in each 

 hemisphere, mounted upon downwardly-directed stalks, and 

 also as to the cylinder 'of nervous matter passing forwards 

 to end abruptly upon the front of the brain. He further 

 agrees with Rabl Rilchkard that the giant nucleus of Leydig 

 is the optical section of this nervous cylinder. In the 

 mole-cricket Dietl describes only one mushroom body on 

 each side, and the stem passing downwards from this is said 

 to divide into two parts, one of these ending in the 

 middle line, whilst the other forms the cylinder ending 

 upon the front of the brain. The various histological ele- 

 ments are described in detail, as they are found in the 

 various parts of the brain. Dr. Dietl finds the nervous 

 matter in invertebrate brains under the three following 

 conditions : — 1. Ganglionic cells, as they are called, and 

 allied structures, free protoplasmic nuclei. 2 Nerve-fibres 

 of the most different sizes. 3. " Marksuhstanz,^'' a peculiar 

 arrangement of nervous matter, which appears sometimes as 

 fine fibrillse, with an axial arrangement, sometimes as a very 

 fine network of different thicknesses, and sometimes as thin 

 lamellae, or altogether homogeneous. Under all these forms 

 this third group of textures is characterised by turning 

 very dark under the influence of osmic acid, whilst the 

 other elements are only turned brown. 



Another valuable addition to our knowledge of insect 



