250 



SKELETON 



DEVELOPMENT OF THE SKELETON. 1 



In the following brief account of the development of the skeleton only the 

 more general features of the early stages will be considered. The details of 

 ossification and many points which relate to the morphology of the parts will 

 be dealt with in other parts of this work. 



The vertebral column is developed from the mesenchyme which invests 

 the notochord and neural canal, and is derived from the sclero tomes. This is also 

 the blastema from which the membranes investing the spinal cord and the 

 ligaments of the vertebrae are produced. The appearance of the skeletal elements 

 is preceded by a stage in which a series of paired cellular thickenings is laid down 

 in the mesenchyme. It has been shown bv v. Ebner and others in the reptilian, 

 and by 0. Schultze, Weiss, and others in the mammalian embryo, that each sclero- 

 tome is divided into a cranial and a caudal portion by a narrow transverse cleft 



(fig. 304). This appears in 

 man in the thoracic region 

 about the end of the third 

 week (Bardeen).' 2 The inter- 

 vals between the sclero tomes 

 disappear, and the dividing 

 cleft is soon obliterated, but 

 the two portions remain dis- 

 tinguishable owing to the fact 

 that the caudal half consists 

 of more densely cellular tissue. 

 The mesenchyme immediately 

 round the notochord loses all 

 trace of segmental cleavage, 

 and becomes condensed into 

 a continuous notochordal 

 sheath. As Weiss has shown 

 in the rat, the myotome 

 extends as a keel-shaped 

 thickening into the cleft, and 

 pushes the two portions of 

 the sclerotome apart, so that 



the caudal portions of each sclerotome pair come to lie obliquely, embracing on 

 their mesial aspect the cranial portions of the succeeding pair. The thickenings 

 are slowly pushed into the intervals between the myo tomes to which they 

 properly belong and the succeeding pair, and thus ultimately come to have an 

 intersegmental position (Bardeen). It must be remembered that these thickenings 

 are merely areas of condensation in a general blastema ; but it is customary 

 to speak of them as primitive vertebrae (or scleromeres, Bardeen). Each scleromere 

 (fig. 305 A) has a pair of dorsal or neural processes (primitive arch), a pair 

 of ventral or costal processes which extend outwards between the myotomes, 

 and a mesial plate which is continuous with the condensed mesenchyme of 

 the notochordal sheath. These plates do not form the future bodies of 

 the vertebrae ; they occupy rather the position of the future intervertebral 

 discs. Between the intervertebral plates the notochordal sheath is invested by 



1 The literature of the development of the skeleton will be found collected by Braus (skeleton of 

 limbs), Schauinsel (vertebral column, ribs, and sternum), and Gaup (skull), in Hertwig's Handbuch 111. 

 Th. ii. and iii. pp. 331 seq., 562 seq., and 855 seq. 



2 Amer. Journ. of Anat. iv. 1904. 



FIG. 304. HORIZONTAL LONGITUDINAL SECTION OF THREE 



PROTOVERTEBRtf: IN A SNAKE-EMBRYO. (v. Ebner.) 



ep, cutaneous ectoderm ; e.m., outer wall of segment ; /, its 

 margins folded round into i.m., muscle-plate composed of 

 flattened cells which are becoming elongated into muscular 

 fibres ; n.c., neural canal, in outline only ; n'.c'., neural 

 ectoderm forming its walls. Between these and the muscle- 

 plate is a continuous mass of mesenchyme which has been 

 derived from the inner parts of the primitive segments, partly 

 interrupted by the ganglion-rudiments, gl. The original 

 intervals between the primitive segments here are still indicated 

 by vessels, v. i.cl., cleft in the mesenchyme (according to 

 Ebner this is the remains of the original cavity of the 

 segment). 



