THE STERNUM : ITS EARLY DEVELOPMENT AND OSSIFICATION. 25 



dates at which the several centres appear. The first nuclei 

 appear generally in the sixth month for the pre-sternum and 

 the first piece of the meso-sternum. In the seventh month 

 centres appear for the second and third pieces of the meso- 

 sternum. No further addition to the number of centres occurs 

 during the eighth and ninth months ; and in full-time foetuses 

 (of which 122 examples were examined) the fourth piece of the 

 meso-sternum and the meta-sternum are usually cartilaginous. 



(B) Position and number of centres of ussification. — In Table 

 II. a summary is given of the situation and number of centres 

 of ossification in the component parts of the sternum. A more 

 intimate analysis did not appear necessary ; and for the sake of 

 brevity and clearness, reference has been omitted to the 

 separation or fusion of double centres, and to obliquity in their 

 position (such oblique double nodules being grouped as lateral). 

 The results tabulated, it should be noted, are reached from an 

 examination of specimens of which the large majority are full- 

 time foetuses. The cases of younger sterna are not only fewer 

 in number, but are also less advanced in ossification. 



While there is considerable variability both in number and 

 position, the centres of ossification are, as a general rule, single 

 and median, and are six in number ; one for the pre-sternum, 

 one each for the first four elements of the meso-sternum, and 

 one for the meta-sternum. Exceptions to this rule are found in 

 all situations except in the meta-sternum, which in the few 

 cases in which bone formation has begun, is invariably provided 

 with a single median centre. 



The pre-sternum is generally formed by a single centre (79 

 per cent.). When there are two centres, they are generally 

 vertically placed. Three or more centres occur quite rarely 

 (2*6 per cent.). 



The first centre for the meso-sternum is, as a rule, also single 

 (78-5 per cent.) ; when there are two, they are usually laterally 

 placed. The second and third centres of the meso-sternum are 

 less frequently single than the above-named centres (59'3 per 

 cent. : 60-5 per cent.). When there are two centres, they are 

 in most cases laterally placed. The fourth centre in the meso- 

 sternum is not generally present. When it occurs it is nearly 

 as often double as single ; and when two centres are present, 

 they are as often vertical as lateral in position. 



