307 



be partly calcified, while bone has been formed, uniting with lliis 

 calcified area. 



So while the first beginning of (he spina is formed by perichon- 

 dria! bone in the mesenchyma between M. supra-, and infraspinatus, 

 its further development is effected by chondria! bone, whicii origin- 

 ates in the younger cartilage. This cartilage has been generated 

 between tiie afore-said muscles by the same mesenchyma. 



A peculiar feature is still to be observed at the shoulder-blade of 

 the embryo of 90 mm. Bone is developed at the margo superior as 

 well enchondrially as perichondrially. In the mesenchyma that forms 

 the perichondrial bone, and into which this bone extends over some 

 distance, there are two cartilaginous nuclei, made up of the same 

 young tissue from which the cartilage of the spina has been built 

 np. Fig. 11 shows in cross section these nuclei, which are not in 

 contact with the remaining cartilage of the shoulder-blade. These 

 cartilage-islets appear to be already calcified and ossified here and 

 there. It is impossible to draw a boundary-line between the bone 

 formed in this process and the perichondrial bone of the scapula. 

 This ossifying process, in which (l)esides the enchondrial ossification 

 of the scapula) both |)erichondrial and chondrial ossification of 

 a cartilage nucleus, situated outside the perichondrial bone, are 

 present, agrees completely with the formation of the spina scapulae. 

 This is strilting, since the spina scapulae and tlie definitive 

 margo superior are the two parts of the shoulder-blade, which are 

 missing in the first rudiment of the cartilaginous scapula. This 

 deficiency vertebral of the place destined for the future incisure, is 

 indeed accounted for by the fact that the margo superior in young 

 embryos is still straight and displays no incisure. The missing parts 

 are apparently supplied by the perichondrial bone that reaches far 

 info the mesenchyma, together with the bone formed by the afore- 

 said cartilage-nnclei. At the shoulder-blade of an embryo of 120 mm- 

 in length, in which the ossification had considerably advanced, the 

 incisure was indeed present. 



Of course, the question arises, how the cartilage of tlie spina as 

 well as the cartilage nuclei are further developing. In both places 

 the cartilage is soon transformed completely into bone. In an embryo 

 of 120 mm. only a very few remnants of the cartilage of the spina 

 were still left. The rest had been ossified. 



After this the development of the shoulder-blade proceeds in the 

 way de.scribed in the text-books of embryology. 



Now let us review once more the current opinions of the develop- 

 ment of the spina scapulae. It will be seen, then, that however 



