508 L. BOLK 



the obliteration appeared in 2| per cent. As I pointed out, 

 there are two possibilities. The process is either confined to a 

 definite period of development, or it can happen during its 

 whole course. To determine which of the two possibilities 

 really occurs, we have only to observe the frequency of pre- 

 mature obliteration appearing in the two groups of youngest 

 skulls, containing those of children from 3 to 6 years. Their 

 total number is 1125. Amongst these skulls there were 24 with 

 partial or entire closure of the sagittal suture, amounting to 

 2.1 per cent. The conclusion therefore is quite simple and lies 

 close at hand. 



Amongst 1820 skulls of non-adult individuals (aged 3 to 20 

 years) there are found 47, or 2.5 per cent, in which the parietal 

 bones are united, and amongst 1125 skulls of children, less than 

 7 years of age, I count 24, or 2.1 per cent, in which coalescence 

 had taken place. Consequently the number of skulls with 

 synostosis of the sagittal suture scarcely increases after the 

 seventh year. 



The period during which the obliteration of this suture in in- 

 fancy begins reaches a lunit therefore in the seventh year. The 

 tendency to premature closing is not extended over the whole 

 period of growth, but practically stops after the seventh year. 

 I recall the fact that exactly the same relation was found in 

 the masto-occipital suture. 



Referring to the suture just mentioned, still another cir- 

 cumstance presents itself, proving that the number of prematurely 

 closed sutures do not augment after the seventh year, i.e., the 

 proportion between 'the partially and totally closed sutures. 

 The former duninish as the skulls reach a more advanced age. 

 To demonstrate this I beg the reader to look at the last two 

 rows on table 3. In Groups I, II and III (skulls up to 8 years 

 of age) the partially closed sutures exceed in number those en- 

 tirely closed; in the Groups IV and V (skulls up to 9 and 10 

 years of age) an equal number of each is found, and finally in 

 Groups VI and VII the entirely obliterated surpass the partially 

 closed ones. I may conclude, therefore, that the process once 

 commenced is of a progressive nature. 



