The studies were carried out on the hypothalamus of 5 newborn (P0 stage) guinea pigs. The sections were impregnated according to three modifications of the Golgi technique or stained according to the Nissl and Klüver-Barrera methods. On the basis of the shape and size of perikarya, dendroarchitecture, pattern of axon as well as the inner structure of neurones, in the medial (Mm) and lateral (MI) mamillary nuclei four morphological types of nerve cells were distinguished: cap-like with two

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... es (33% of the cell population), fusiform (35%), triangular (12%) and rounded unidendritic (21%) neurones. The majority of them possessed spines on their dendrites. The spiny cells, both cap-like and fusiform ones, were observed preponderantly, in the medial mamillary nucleus, whereas in the lateral mamillary nucleus there were mainly seen the triangular and fusiform neurones, either spiny or aspiny cells. The spineless rounded unidendritic cells were dispersed throughout the mamillary region, but they were twice as numerous in Mm as in MI, where they were the least numerous.