Variations and Malformations
In our collection we have observed an unsually high number of developmental abnormalities. We believe that this high incidence is related to the fact that our collection consists exclusively of spontaneously aborted embryos and fetuses. The highest number of abnormalities was seen at the level of the spine. In an earlier report  we described 11 malformations in 266 specimens. Since then we have found five more vertebral malformations. In all but one specimen the malformations were observed before the process of ossification had started. Our observations are limited to the level of vertebral bodies. Malformations are divided into three types: type 1, failure of formation; type 2, failure of segmentation; and type 3, mixed malformations . Type 1, failure of formation, is subdivided into defect of formation and error of formation. Type 2, failure of segmentation, is similarly subdivided into defect of segmentation and error of segmentation. Defect denotes either an incomplete formation or an incomplete segmentation. Error, on the other hand, is characterized by an abnormal formation or by an abnormal direction of segmentation. Type 3 groups all malformations in which failure of formation and failure of segmentation are observed simultaneously. An example of a defect in formation is a hemivertebra (Figs. 15.1, 15.2) whereas an abnormally shaped vertebra represents an error of formation (Fig. 15.3). A defect of segmentation can be seen as an unsegmented bar (Fig. 15.4) or a block vertebra (Fig. 15.5). A V-shaped segmentation leading to an oblique intervertebral disk space (Fig. 15.6) represents an error in segmentation. Type 3, representing the simultaneous presence of failure of formation and failure of segmentation, can be seen in Figs. 15.7 and 15.8.
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