224 D. DAVIDSON BLACK 



and the average size of its cell elements become progressively 

 smaller from without inwards. The fourth or polymorphic 

 layer (IV) in the figure appears to be further subdivided into a 

 more superficial zone of scattered small cells and a deeper more 

 compact zone This is not the case, however, and this appear- 

 ance is due to some of the elements in this layer being arranged 

 in groups of irregular size, the cells in which are more closely 

 packed than in the intervening spaces. Only a portion of one of 

 these groups appears in the drawing and included in it are shown 

 several very large well developed pyramidal cells. These large 

 pyramidal cells occur singly or in groups over this area and are 

 situated at about the level of the middle trisection of the poly- 

 morphic layer. The whole depth of this layer is not shown in 

 the figure. The cells referred to exceed in size the largest cells 

 found n the normal foetal cortex used as control. 



Figure 16 is taken from the so-called anter or lobe of the cere- 

 bral vesicle at a point about an inch from the arched margin of 

 the same in the mid-line. The plexiform layer (I) is here quite 

 sharply marked off from the subjacent cell layer. In this layer 

 (II) the elements are somewhat closely packed and arranged in 

 irregular groups. It varies considerably in thickness at differ- 

 ent points at the expense of layer III. In the latter layer the 

 arrangement of cells is somewhat looser and it apparently cor- 

 responds to Bolton's fourth layer or "nner fiber lamina (2). Layer 

 IV is made of closely packed small cells having a quite character- 

 istic embryonic arrangement in the form of irregular rows at 

 right angles to the surface of the cortex. The line of demarca- 

 tion between this layer and the preceding one is very easily made 

 out and in places is a' most as sharp as that between the plexi- 

 form layer and layer II. Layer V, the whole thickness of which 

 is not shown in the figure, is the thickest of the cortical laminae, 

 and combined with layer IV makes up more than one-half of 

 the total thickness of the cortex. Its eels are arranged in a 



Fig. 15 Cyclopian cortex cerebri. Section taken from the region of junction 

 of the cerebral vesicle and the thalamus. Explanation in text (page 223). X65. 



Fig. 16 Cyclopian cortex cerebri. Section taken from the 'anterior lobe.' 

 Explanation in text. X 65. 



