Brain Struct Funct (2011) 216:301–317 as the merger of two layers that also characterize the adjacent subiculum: a deep layer containing large pyra- midal cells and superﬁcial layer composed of small pyra- midal cells. Lorente de No´ (1934) instead focused on spatial distribution: ‘‘…. There are two types of pyramids, superﬁcial and deep ones. The superﬁcial are arranged in one or two very dense rows. The deep pyramids are grouped into several less dense rows below …. It is important to distinguish both types of pyramids for two reasons. One is that the deep pyramids are less numerous in lower mammals (mouse, rabbit, dog, cat) than in primates (monkey, man).’’ His second reason, pertaining to con- nections between pyramidal cells and basket cells, will be discussed in the section on afferent connections. Phylogenetic differences in the distribution of pyramidal cells were also recognized by Stephan (1975) in his extensive review of the allocortex. Based on the increasing number of deep pyramidal cells in species which may represent stages of primate evolution from insectivores to humans and the concomitant dissolution of the compact, superﬁcial cell rows, Stephan suggested a progressive ‘‘invasion’’ of the stratum oriens by pyramidal cells, which, in a hypothetical ancestor, were located in a compact cell layer. This appealing phylogenetic interpretation of CA1 cytoarchitecture in primates (e.g. human, Fig. 2a; marmoset monkey, Fig. 2b) has somewhat distracted attention from the fact that a substantial portion of stratum oriens is occupied by pyramidal cells in species as taxonomically diverse as duck-billed platypus (Ornithorhynchus anatinus, Rose 1926), the small Australian marsupial Parma wallaby (Macropus parma, Fig. 2f) and common brush-tailed pos- sum (Trichosurus vulpecula, Fig. 2g), the Guaira spiny rat (Proechimys guaira, Scorza et al. 2011), pig (Sus scrofa, Schaffer 1892; Holm and Geneser 1991), dog (Canis lupus familiaris, Rose 1926; Hof et al. 1996), fox (Vulpes vulpes, Fig. 2h), and bottlenose dolphin (Tursiops truncatus, Jacobs et al. 1979). In these species, spanning all mammalian subclasses and representatives in major orders, the ‘‘inva- sion’’ of stratum oriens is not accompanied by a dissolution of the denser, superﬁcial rows of CA1 pyramidal cells. A cytoarchitectural presentation of CA1 similar to that seen in primates can be found in microchiropteran bats. While some condensation of superﬁcial pyramidal cells is still vis- ible in the large-eared slit-faced bat (Nycteris macrotis, Fig. 2c) a fairly homogeneous distribution of cells can be seen in the little free-tailed bat (Chaerephon pumilus, Fig. 2d). While many similar examples can be found among microchiropteran bats (I. Amrein, unpublished observations), CA1 is split into two fairly compact layers in the Burmese whiskered bat (Myotis montivagus, Baron et al. 1996) resembling the appearance to CA1 in reeler mice (see below, Development). A bilaminar CA1 is also seen in the common mole rat (Cryptomys hottentotus, Fig. 2i), a member of the 303 mole rat family (Bathyergidae). In contrast to microchiropt- eran bats, a rather wide but otherwise inconspicuous CA1 is found in some megachiropteran fruit bats (Epomorphus wa- hlbergi: Fig. 2e, Rousettus amplexicaudatus: Baron et al. 1996, Pteropus scapulatus: Buhl and Dann 1991). Cytoarchitectural differences between the mentioned species can be understood as an ‘‘invasion’’ of stratum oriens in which either only deep pyramidal cells or both deep and superﬁcial pyramidal cells participate. Deep and superﬁcial pyramids seem, to some degree, to be indepen- dent in their participation in this process. It is also note- worthy that two sublayers, which differ in the cell sizes and packing density, can still be identiﬁed in the human CA1 (Fig. 2a, Braak 1974) although it may require unusually thick sections to ascertain small differences in cell densities. Braak (1974) applied the terms ‘‘stratum superﬁciale’’ and ‘‘stratum profundum’’ to these sublayers, which also have been rendered visible in diffusion tensor images of the human hippocampus (Shepherd et al. 2007). A laminar division is more clearly visible in the part of the monkey pyramidal cell layer identiﬁed as CA2 by Rosene and van Hoesen (1987). Also, in one case (11/16) reported by Slo- viter et al. (2004), an astounding subdivision of the human CA1 pyramidal cell layer into two tiers was observed. Can two sublayers be identiﬁed in all species and, in particular, are they present in mouse and rat? CA1 pyra- midal cells form a compact layer without an apparent subdivision throughout much of the septotemporal extent of the eastern rock elephant shrew (Elephantulus myurus, Fig. 2j), chinchilla (Chinchilla laniger, Fig. 2k), guinea pig (Cavia porcellus), and rabbit (Oryctolagus cuniculus) hippocampus (Geneser-Jensen et al. 1974; Geneser 1987). A similar appearance is seen in approximately the septal one-third of the rat (Fig. 2l) and mouse hippocampus (Fig. 2o). The division into deep and superﬁcial layers that becomes apparent further temporally can, however, be identiﬁed in fortuitous sections (Fig. 2m) and, more regu- larly, in the distal part of the septal CA1. Further tempo- rally, an easily visible sublayer of loosely arranged pyramidal cells becomes ﬁrst apparent distally. Two his- tologically distinct sublayers are visible along the proximo- distal extent of CA1 (except for a very narrow sector at the proximal limit) throughout most of the temporal two-thirds of the hippocampus of the rat and mouse (Fig. 2n, p). A distinction of two sublayers in CA1 as a characteristic of some parts of the mouse hippocampus was recognized by Rose (1926), and a division of the layer was also mentioned by Ramo´n y Cajal (1893). In the rat, the division appears sufﬁciently striking in Nissl-stained preparations to be illustrated throughout the septotemporal extent of CA1 in the atlas of Swanson (1992), but not in the widely used atlas of Paxinos and Watson (1986). Also, a radial division of the layer was not mentioned in the germinal works of 123