Scientia Sinica

134 SCIENTIA SINICA Vol, V

RESULTS “AND Discussion

The ossification centre of the zygomatic bone first appears in a fetus of 34 mm C-R length, aged 59 days according to estimate (fig. 1). Mall®! described the centre of the zygomatic bone appearing in a fetus of 32 mm C-R length, estimated at 57 days. Thus the date of appearance of the ossification centre in the present study seems to be two days later than that given by Mall. However, the age of Mall’s specimen was estimated in days by multiplying the square root of the CR length in millimetres by 10 or VC-R(mm) 10. In the present study, the age was determined according to Patten’s table. Mall’s specimen of 32 mm CR length in Patten’s table gives an age of about 58 days. Thus the actual difference in the dates of appearance of the ossification centre of the zygomatic bone in both studies is only one day. No second centre of ossification was seen in all our specimens.

The zygomatic bone in adult skull is sometimes seen to be divided transversely into 2 or even 3 bones. The possible explanation is that in rare cases the zygomatic bone may be developed from 2 or 3 centres of ossification, but, in general, it is ossified from a single centre only.

The time of appearance of the ossification centre and the differentiation of the different processes of the zygomatic bone are given in Table 1.

Table 1. Time of Appearance of Ossification Centre and Different Processes of the Zygomatic Bone

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No. C—R length Age Appearance of Ossification Centre and Processes 80 10 5 None 26 12 6 3 52 12 6 > 44 15 6 » 51 15.5 6 > 55 16 6 ” 31 18 6 » 49 21 7 33 53 27 7 » 18 29 8 i 20 31 8 3 48 31 8 a 17 34 8 Ossification centre 54 37 8 Inf. & zygo. proc. 46 | 42 9 Fron. proc. 5 45 9 $5 53 25 53 10 Orbit. proc.

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