4 SHOULDER-GIRDLE AND BREAST-BONE. 



These two skeletons maintain their greatest independence of each other in the oldest and 

 most generalised types of Fishes ; in the newer (cycloid and ctenoid) types the correlation of the 

 two becomes much more evident and perfect ; but we must pass through all the cold-blooded 

 vertebrates, and even past some of the lower types of the warm-blooded, before we see the 

 greatest inter-dependence of these diversely developed structures. 



In working out the morphology of the Shoulder-girdle and Sternum, the various modes of 

 ossification have cost me much histological labour ; taking counsel on this subject, 1 I have deter- 

 mined to call that ossification which commences in the intercellular substance of hyaline cartilage 

 " endostosis." 



That bony matter which is first found in the almost structureless, inner layer of the peri- 

 chondrium, in immediate contact with the outermost cartilage-cells, is formed by a process which 

 may be called " ectostosis." 



Such a bony formation as appears primarily in the skin, in the subcutaneous fibrous mesh, 

 or in the aponeurotic tracts, may be called " parostosis." 



All these modes have to be most carefully traced backwards to their earliest appearance, 

 otherwise the study of osseous centres is most perplexing ; for one of the constant results of 

 ossification in the higher classes is the coalescence of parts morphologically separate. 



Nothing, however, can exceed the study of segmentation in importance and interest. Without 

 a clear notion of that process in all its modes and degrees, the anatomist's ideas will keep in a 

 hopelessly confused state. 



Cartilage, as every one knows, is a very fast-growing tissue ; tending to develop itself in an 

 unstinted manner over many morphological regions at once, and only showing the proper terri- 

 tories by the law of segmentation, which law is potent in very various degrees in different parts, 

 and in the corresponding parts in different types of vertebrate creatures. . 



Fission of, or the formation of two or more osseous centres in the same tract of, hyaline car- 

 tilage are the two kinds of segmentation to be observed ; but they both have the most varied 

 degrees of development. 



If a fissure be open at one end only in a plate of cartilage it is called a " notch ;" if closed 

 at both ends, and dilated so as to form an oblong, oval, or round space, it is here called a 

 " fenestra ;" when the cleft is complete we have a true joint ; this may have a " synovial cavity," 

 or be merely a fibrous tract the lowest kind of joint. 



All these open spaces in cartilage are the result of development, and may be called 

 secondary in relation to those which are to be found at the mid-line, both above and below ; and 

 which depend upon imperfect coalescence of symmetrical cartilages. 



Both the Sternum and the Shoulder-girdle have these primary notches and fenestoe, which 

 are not to be classed amongst clefts, but agree with the upper and lower " fontanelles" of the 

 primordial skull. 



Morphological territories are also measured out in another way than by the formation of 

 perfect, or of more or less imperfect clefts ; namely, by two or more centres of ossification appear- 

 ing in an undivided tract of cartilage. 



We are all familiar with this nascent segmentation in the limbs and limb-girdles of Man 

 and the Mammalia generally ; in the long bones the chief part or shaft commences to ossify very 



1 With Professor Huxley. 



