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--."'1"'1I""': During the development of the cerebellum, mechanisms as cellular proliferation in the 1 cortexor betweenthe cortexandthewhrtematter (Mares and Lodin, 1970 a, b), piamater and cortex relation (Sievers et ai, 1987), genetic factors (Wahlsten and Andison, 1991) and changes in the deepest part of the cerebellar cortex (Goffinet, 1983, PenaMelian, 1986) have been hypothesised as having a role in fissuration. However, the intimemechanism of this process still remains unclear. ~ is well known that during development of the cerebellum in chick (Feirabend, 1990) and cetacea (Korneliussen, 1967) there is a period when four clusters of first order Purkinje cells (PC) are observed in the'inner cortical cell layer (ICCl). Each PC cluster relates with each of the four cerebellar nuclei, a~hough the role of these clusters in the IGCl has not yet been well established. The aim of this work has been to find a relationship between the clusters of first order PG and the appearance of primary fissures (prima, praepiramidalis, secunda and sulcus uvularis-1) in the cerebellum of the avian embryo. 34 HH (Hamburger and Hami~on, 1951) to 37 HH chick embryos and 23 Z (Zacchei, 1961 )10 26 Z quail embryos were fixed in 4 % paraformaldehyde and embedded in wax. Transversal and sagital sections were stained with Hematoxilyne-Eosyne or Bodian, dehydrated and mounted in Eukytt. Some of the sections were drawn in light chamber for reconstructions. The cerebellar surface of 34 HH chick embryos does not show any fissures. However, ICGl is divided in several plaques (usually four) consisting of primary order PC clusters. These dusters are very close to each other, sometimes overlaping, as in the cerebellar top (Fig. 1). When fissuration starts, at 35+ HH in the avian embryo (Pena-Melian, 1986), the gaps between the IGGL plaques underty each fissure, and convergence of both two plaques is seen (Fig. 2). Once the fissures are established (36 HH in chick embryo) the cerebellum has one dorsal fissurated zone and one ventral not fissurated zone. The ICCl is a continuum in the fissurated zone, while in the not fissurated one the previously observed fissurated pattern is seen (Figs. 3 and 4). If the gaps amongst plaques are projected towards the dorsal surface, they coincide with the fissures (Fig. 4), which suggests that the appearance of the first fissures (prima, praepiramidalis, secunda and sulcus uvularis-1) occur over the space between the IGCl plaques, a~hough they fusse afterwards. Our observations suggest that initial fissuration -....

Role of the cerebellar inner cortical layer in the onset of fissuration in the avian embryo.