122 Dr. E. Schuster. Cell Lamination of the [Sept. 30, 



In Section C the same general arrangement is seen, but the strip of 

 Type V has become narrower and of Type IV broader. In Sections A, B, 

 and C the upper boundary of Type IV is formed by fpsa. A short way 

 behind Section C this fissure changes its direction, running more vertically 

 upwai'ds. Cortex of Type IV runs upwards as a narrow tongue behind it. 



The Nuvibers of Nerve Fibres in the Cranial Nerves of Echidna. 



It was suggested to me by Dr. Mott that the numbers of fibres in the 

 cranial nerves might offer a useful indication of the relative development of 

 different parts of the cerebrum, and consequently the piece of work described 

 here was undertaken. 



The material available was the brain from which the foregoing description 

 of the cerebral cortex was made, and a brain of Echidna setosa which had 

 been brought back by Mr. Geoffrey Smith from Tasmania. The latter was 

 not in a good enough state of preservation to be used for any other purpose 

 but the counting of the fibres in the cranial nerves. 



Nerves III, IV, VI, VII, IX, X, XI, XII were small enough to be viewed 

 in one field of the microscope under a mag-nification great enough to see each 

 fibre in the stained transverse section separately, or, when not small enough, 

 they were divided into separate nerve bundles, each of which could be viewed 

 as a whole. The counting of the filjres was in these cases a simple and 

 direct, though somewhat laborious matter. A plan was made of each nerve 

 by means of the " Abb^ " drawing apparatus, a dot or circle representing each 

 nerve fibre. The dots or circles were then carefully counted. 



Nerves II, V, and VIII were too large in transverse .section to be treated in 

 this way, and were not conveniently divided up by connective tissue. Another 

 method had therefore to be adopted. The transverse section of the nerve 

 was placed under a power just low enough for it to be entirely included in 

 the field of the microscope, and its outline was traced by means of the 

 drawing apparatus. Then, without shifting the paper or drawing apparatus, 

 or changing the lenses of the microscope, a Zeiss object netz-niicronieter was 

 substituted for the microscope slide bearing the section, and the small squares 

 ruled on this apparatus were traced on the top of the drawing. Each of 

 these scpiares lias sides 1/20 mm. in lengtli, and therefore an area of 

 1/400 sq. mm. By countiug the number of squares included in the outline 

 of the nerve, a fairly accurate estimate of the area of the transverse section 

 can be obtained, which is not in any way aifccted by the distortion produced 

 Ijy the camera lucida. 



A sulliciently high power was then put on the microscope and the outline 

 of a convenient number of squares traced on a fresli piece of paper; the 



