1909.] 



Cerebral Cortex of Echidna, etc. 



121 



2. A dense layer of large, darkly staining cells, measuring up to 35 x 20 n. 

 These cells are characterised by the strong development of their dendrites, 

 two of which are often directed towards the surface. 



3. At 0"G mm. below the surface, or just above, these cells give place to 

 others, distinguished from them by being slightly smaller and irregularly 

 pyramidal or pyriform in shape, with only one apical process. This layer 

 extends downwards to depth of rather less than 0"9 mm. 



4. A stratum almost devoid of cells, about 0*1 mm. broad. 



5. At a depth of 1 mm. a layer of polymorphic cells may be found, which 

 measure about 20 x 15 ^ti,. Cells of this type occur throughout the rest of the 

 •cortex, but at a depth of 1"2 mm. they become less numerous and are partly 

 replaced by numbers of small stellate cells. 



6. Thus a sixth layer can be distinguished, characterised by the presence of 

 the latter. It stretches from 1'2 to 1"6 mm. from the surface. 



Distribution of Cortex of Type IV and Type V. 



Cortex of Type IV may be seen in Section A, lying between the fissura 

 rhinica posterior {frlip) and the fissura postsylvia anterior (fpsa). In this 

 section the fissura rhinica anterior (frha) is cut in a plane more or less 

 parallel to the direction in which it runs but at right angles to its walls. 

 The walls are thus shown in section near to the bottom of the fissure. It 

 will be seen that the cell lamination of the lower wall is of Type IV. In 

 a section taken a little in front of this the whole length of frha is cut in 

 this way and the fissure is shown opening on to the lateral surface of the 

 hemisphere, but in such a section the lower wall is made up, at any rate near 

 its outer end, of cortex of Type V. In Section A the latter type of cortex is 

 seen lying on the mesial side of frhp. In this region and anterior to it, it is 

 of a slightly different structure to that illustrated in fig. 6, which was taken 

 from Section B. The cells of the outer layer are smaller, not so numerous, 

 nor so rich in dendrites. In Section B it will be seen that Type V has left 

 the region of frhp and only covers the part of the rhinencephalon lying next 

 to the hippocampal fissure {FIT). Meanwhile Type IV has encroached 

 beyond frlip, which in Section A formed the boimdary between the two 

 types. Types IV and V do not pass directly into one another, but are 

 separated by a broad strip of cortex more or less intermediate in structure. 

 This lacks the most characteristic feature of each of the two types which it 

 separates, namely the layer of granules of the one and the large, darkly 

 staining cells in the outer layer of the other. The latter do not stop 

 suddenly nor die out gradually, but they become first reduced in numbers, 

 then arranged in groups separated by free spaces and then finally disappear. 



