The rapid development of molecular biology and genetics has led to renewed interest in embryology, comparative embryology, and studies of the relations between ontogeny and phylogeny. In fact, genes have been identified which are involved in the formation of shapes and structures, and it is becoming apparent that their primary morphological expressions are conspicuously similar in different species. The primarily identical shapes do not become diversified until advanced individualization of embryos, and it is here that it is possible to employ the knowledge of comparative embryology, the branch of science engaged in the study of the development and differentiation of tridimensional structures in different animal groups. However, comparative embryology has been neglected during the past decades, as its development has appeared to have been completed. In our opinion, the decreased interest in comparative embryology has been caused by the fact that often the time factor was not or could not be respected. In fact, in the case of embryos of wild animals even their ontogenetic age and sometimes the duration of intrauterine development are unknown.

Morphogeny of the nasal skull was investigated in 92 embryos of 13 species of Cetacea. Compared to the original nasal capsule of land mammals, the nasal structures of Cetacea show many weighty transformations. As a result, the nostrils are translocated from the tip of the snout to the vertex of the head. Several structures of the embryonic nasal skull remain preserved even in adult cetaceans. The translocation of the nostrils to the highest point of the surfacing body is among the most perfect adaptations of cetaceans to the aquatic life habits. The morphogeny of the nasal skull suggests that all cetaceans are of common origin and form a single monophyletic order. The hitherto usual division of this order into two suborders, Mysticeti and Odontoceti, appears to be unsubstantiated. Rather, at least three closely related superfamilies should be distinguished within the order Cetacea, viz., Balaenopteroidea, Physeteroidea, and Delphinoidea. The results are in a

The development of the breast-shoulder apparatus in the Marsupialia was inves tigated and compared with the conditions in Monotremata and Placentalia. The results were achieved by the investigation of material comprising altogether 109 histological serial sections of intrauterine embryos, neonates, and pouch young from 11 marsupial species. Additionally, 54 skeletons of subadult and adult marsupials from 25 species were included for comparison. The embryonic states show a strong similarity to the developmental stage of the breast-shoulder apparatus in the monotremes. In contrast, the adult breast-shoulder apparatus generally corresponds to that in placentals. The following elements can be observed in the marsupial breast-shoulder apparatus during embryogenesis: scapula, metacoracoid, procoracoid, first rib, paired sternal elements, unpaired sternal element, and clavicle. All the elements mentioned together form a compact, continuous arch in both the intrauterine embryos and the neonates. In the pouch young, this arch is reduced rather soon after birth, so that a compact connection between the left and the right half of the body no longer exists. All that remains is a loose connection via the clavicle. The metacoracoid becomes the processus coracoideus scapulae. The procoracoid becomes the praeclavium. The unpaired sternal element fuses with the paired sternal element, generating the uniform manubrium sterni. The first rib takes its usual position in the thorax. In the pouch young, the breast shoulder apparatus as a whole already shows all the typical characteristics that can be determined in adults.