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During the Early Pleistocene, two ground squirrel species, Spermophilus praecox (northern Black Sea area) and S. polonicus (Central Europe) coexisted in geographically well-separated ranges. Studie...

Discrete characters of the occlusal surface (additional cusps) have been studied to elaborate a new approach to the identification of the Ground Squirrel species Spermophilus odessanus, S. suslicus, S. pygmaeus, S. citellus, and S. xanthoprymnus. Data on the presence/absence of the additional cusps have been represented as star plots and, in addition, have been studied using discriminant function analysis. The species-specific sets of the characters (patterns of bunodonty) have been revealed and are of high diagnostic value. The Citellus-set is defined by the presence of mesostyles and the rareness of the metastylids, paraconules and metaconules, hypostyles and protostyles. The Pygmaeus-set is characterized by the presence of additional cusps in the lower cheek teeth. The Odessanus-oriented set is found in the Spermophilus pygmaeus, S. odessanus, and S. suslicus. The relatively high frequency of additional cusps of the metaloph and the paraloph is characteristic for this set. The Plesiomorphic-set (characters shared by all the studied species and for this reason regarded herein as ancestral) is found in S. xanthoprymnus. The patterns of bunodonty serve as diagnostic criteria only as a whole: the shape of a star plot (relations among the character frequencies), rather than certain character values, is indicative. An optimal level of identification of species is possible based on the combination of the discrete characters mentioned and on the size parameters of the third upper molar (an occlusal set). The occlusal sets are intended to remain stable during the time of species existence and seem to correspond to trends in specialization. The functional meaning of the sets can be explained by the dependence between the presence/absence of the discrete characters and the shape of the crown and its main lophs. Each pattern is likely to correspond to a trophic niche, and this niche corresponds to the species. J. Morphol., 2016. © 2016 Wiley Periodicals, Inc.

The development of the gonads in male and female chick embryos with induced unilateral mesonephric agenesis was studied using grafting, histoenzymology, and electron microscopy. As in embryos with a mesonephros, proliferation of the coelomic epithelium and its interaction with mesenchymal cells to form the medullary cords take place in the amesonephric gonads. In a similar manner, gonadal sexual differentiation and the differentiation of steroidogenic tissue, detectable by the presence of Δ5-3β-hydroxysteroid dehydrogenase, do not appear to be affected by the absence of an organized mesonephros. However, the initiation of gonadal development, further growth, and the onset of meiosis observable in developing ovaries are retarded. This delay appears to be reversible, as was demonstrated by experiments in which ovaries from chicks with complete mesonephric agenesis were transplanted into the coelomic cavity of male and female 3 1/2-day-old embryos. Meiosis finally occurred in the oocytes of all ovaries, regardless of the sex of the host. Therefore, the presence of a differentiated mesonephros in chick embryos is not required for the establishment of an undifferentiated gonad and sexual differentiation, or for initiation of meiosis.