1909.] 



Cerebral Cortex of Echidna, etc. 



119 



2. A superficial layer of somewhat closely- packed cells of irregular shapes, 

 characterised by being broad relative to their length. The size of these is 

 about 8 to 18 /4 in length and 7 to 12 /i in breadth. About 01 mm. from the 

 surface the cells begin to get less crowded, and at a depth of 0*2 mm. a 

 layer (3), somewhat poor in cells, succeeds. This is about 0'25 mm. broad. 

 What cells are contained in it seem a shade larger than those of layer 2, and 

 they become larger as one goes deeper. They are somewhat irregular in 

 shape, but many are roughly pyramidal. 



4. Layer of large pyramids starts 0'5 mm. from the surface and stretches 

 downwards to a depth of 0'9 mm. The majority of the cells in this layer 

 are elongated and somewhat irregular pyramids measuring up to 30 yu, in 

 length and 15 /x, in breadth, with an average size of 25 x 10 yu. or so. Their 

 nuclei are 7 x 13 or somewhat less. Among these large characteristic cells, 

 which are present in considerable numbers, lie many much smaller cells. 

 These granules extend throughout the cortex except in the most superficial 

 portion, but they are most numerous among and immediately below the large 

 pyramids. 



5. The rest of the cortex is made up of small polymorphic and spindle- 

 shaped cells measuring about 25 x 8 /x, with roundish nuclei 5*7 in 

 diameter. 



Distribution. — Type II may be found on the mesial surface of the 

 hemisphere dorsal to the fissura vallaris. Anteriorly it extends at least as 

 far as that fissure, while posteriorly its limit appears to be the radial fissure 

 continuous with fissura postsylvia anterior (fpsa). Above it spreads over the 

 superomesial border on to the lateral surface, where it abuts on the area 

 covered by Type III. 



Type III. x x x (vide fig. 4). 



Fig. 4 is drawn from a strip on the lateral surface about 5 mm. from the 

 superomesial border, and 3 mm. from the point at which Type II changes 

 into Type III. The depth of the cortex is here about 1'75 mm. 



The following layers may be recognised : — 



1. Molecular. — 0*13 mm. in breadth, or more than double the breadth of 

 the molecular layer of Type II. 



2. Layer of " small pyramids " somewhat elongated in shape, measuring 

 12 to 30 fi, in length x 5 to 10 /x in breadth. 



Although some might correctly be described as pyramids, the majority are 

 not pyramidal, but rather sausage- or spindle-shaped. They often have two 

 apical processes. This layer extends to a depth of about 0-4 mm. 



3. A zone about 0 5 mm. in breadth rather poorer in cells than layer 2. 



