DEVELOPMENT OF THE SKELETON. 



II. DEVELOPMENT OP PARTICULAR SYSTEMS AND 



ORGANS OP THE BODY. 



THE SKELETON AND ORGANS OF VOLUNTAEY 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 tliem. 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 the- 

 process 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, Fig. 525. 



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. 



1. VERTEBRAL COLtJMN AND TRUNK. 



Relation of Vertebral Erudiments to 

 the Notochord. — It has already been shown 

 (General Phenomena of Development, p. 692), 

 that all the parts of the skeleton ow^e their 

 primitive formative material to mesoblastic 

 elements, and that the bodies and arches of 

 the vertebras, and the adjacent part of the 

 cranial walls are formed from continuous 

 blastodermic substance lying below and around 

 the primitive medullary canal. A part also of 

 the basis of the cranium has this in common witli the vertebral axis, 

 that its formative substance surrounds the notochord, extending for- 

 ward from the column of the vertebral bodies into the occipito-sphenoid 

 part of the cranial basis, which is there composed of the formative 

 substance termed the invesiing mass of Rathke. 



It is to be remembered, however, that closely as the formative tissue 



