Book - Quain's Embryology 5

The Skeleton and Organs of Voluntary Motion

The morphological development of the skeleton and organs of voluntary motion is closely in accordance with the general plan of development which belongs to the whole vertebrate body. The first steps are connected with the formation of the strictly axial part, consisting of the enclosing walls of the cranio-vertebral cavity for containing the rudiments of the brain and spinal marrow, and for the issue of the successive pairs of nerves arising from them. These are succeeded by the formation of the walls of the great visceral cavities of the head and trunk, in which the facial and costal arches are to be distinguished ; and lastly, the appendicular parts, or the limbs and limb-arches, are developed. The permanent forms of these parts are only produced in the process of ossification ; but the rudiments of most of them are already to be distinguished in the masses of cartilage or formative tissue which precede the ossifying change.

As the mode of ossification of the several bones has been described in the osteological part of the work, and the histological view of theprocess of formation of bone has been given in the part on General Anatomy, the morphological view of the development will alone be referred to in this place, in which will be included the more important phenomena of the preparation of the matrix or formative material for the various parts of the skeleton.

Fig. 525. — Embryo of the Doo seen from above wIth a Portion of the Blastoderm attached.

The medullary canal is not yet closed, but shows the dilatation at the cephalic extremity with a partial division into the three primary cex'ebral vesicles ; the posterior extremity shows a rhomboidal enlargement. The cephalic fold crosses below the middle cerebral vesicle. Six primordial vertebral divisions are visible ; so, the upper division of the blastoderm ; sp, the lower division.