Studies over de Nederlandse fossiele Cetacea
Grondboor & Hamer , Volume 25 - Issue 5 p. 98- 110
The third part of our studies concerns an introduction to the development and growth of the vertebral body. As literature on the early development of the whale embryo is lacking we mainly have to rely on data from comparative anatomy. The formation of the neural tube and its relative position to the chorda dorsalis is described. Around these two structures the vertebrae originate, first as mesenchymal sclerotomes. From these sclerotomes by a rearrangement of their cranial and caudal halves mesenchymal primitive ve r t ebræ are formed; from these by the process of chondrification solid cartilaginous vertebral primordia come into existence. These primordia are transformed to bony vertebrae by enchondral and perichondral ossification. We then have to focus our attention on the growth of the young vertebrae, and especially on the vertebral bodies. We can observe five phases of development. A. The adult vertebral body in which the bony epiphyses are fused completely with the vertebral body. B. The adult vertebral body in which the epiphyses, though fused with the vertebral body, still can be discerned as such. C. The vertebral body in which only the cranial epiphysis is fused wi th the vertebral body, the caudal epiphysis is separate. D. The vertebral body in which both epiphyses are separate. D and to a lesser extent C correspond to the phase of slow growth of the animal. E. The young vertebral body in which the bony epiphyses are not yet in existence. This situation corresponds to the period of fast growth taking place immediately after birth and continuing during six to twelve months depending on the species. The body of these vertebræ is short but has already approximately its adult thickness and adult processes. Consequently a typical young type exists, short, flat caudal and cranial ends, and an angular profile. Ignorance of this phenomenon has caused wrong determinations. See figure 11 for the various types. These types are discussed for different vertebræ from different regions of the vertebral column. The development of the vertebral body can reflect differences between various whale species.
|404023.jpg Cover Image , 26kb|