428 DOMINICK P. PURPURA 



packed apical dendrites of pyramidal neurons and may be followed for 

 100-200 /x before they are lost in a maze of apical dendrites. 



The major maturational change observed in neocortical pyramidal neurons 

 consists in a proliferation of basilar dendrites and axon collaterals (Bishop, 

 1950; Cajal, 1960; Conel, 1939, 1941, 1951; Eayrs and Goodhead, 1959; 

 Purpura et al., 1960a; Schade and Baxter, 1960). In the cat the growth of 

 basilar dendrites of medium and large pyramidal neurons appears to gain 

 momentum within a few days after birth and an accelerated phase of basilar 

 dendritic growth occurs during the second postnatal week. By the end of the 

 third week, basilar dendrites of extraordinary length and large diameter are 

 readily seen in all neocortical areas. 



It is a curious fact that during postnatal ontogenesis the density of dendrites 

 in the molecular layer decreases due to the prohferation of non-neural ele- 

 ments and regression of the elaborate dendritic apparatus of Cajal-Retzius 

 cells (Cajal, 1960; Conel, 1951). With advancing age, lateral branching of 

 apical dendrites in middle and proximal segments increases, but such branches 

 never approach dimensions of 1-2 mm as claimed by Chang (1951). Distal 

 segments of apical dendrites undergo moderate proliferation during the 

 second and third postnatal week, but tangential spread of these elements 

 rarely exceeds 200 /x. 



In summary, it is to be noted that except for the regressive changes in 

 dendrites of Cajal-Retzius cells, relatively few major alterations are seen in 

 the dendritic components of superficial neuropil during postnatal ontogenesis 

 in the cat. This is in striking contrast to the profound maturational changes 

 which occur in the basilar dendritic system and deep neuropil, as has also 

 been noted in other species. The relative decrease in the density of dendritic 

 elements in the molecular layer during the second and third week occurs 

 pari passu with an increase in the number of axons from various sources. 

 This in turn correlates with the appearance of spines or thorns on apical and 

 basilar dendrites. In view of recent electron microscopical data indicating 

 that the spines observed in Golgi material are loci of synaptic contact (Gray, 

 1959), it can be expected that the morphological alterations occurring during 

 the second and third postnatal weeks will be reflected in significant changes 

 in the physiological and pharmacological organization of the sub-pial 

 neuropil of neocortex. Support for this is provided by a consideration of the 

 changing characteristics of synaptic activities generated in superficial axo- 

 dendritic pathways by stimuli apphed to the pial surface. 



Ontogenetic Changes in Evoked Superficial Negative Responses of Neocortex 



Graded superficial cortical responses (s.c.r.'s) to local stimulation can be 

 evoked from all parts of the dorsolateral convexity of the cerebral cortex in 

 near-term fetal cats (Fig. 2). In perinatal preparations, s.c.r.'s recorded 1-5 



