POPULAR SCIENCE 75 



is one of the most difficult points in any theory of the descent 

 of man, and its significance is probably commensurate with 

 its difficulty. The following are the sahent facts. Man differs 

 from all the other mammalia in the fact that the upper incisor 

 teeth are carried by the maxilla instead of by a separate bone 

 named the inter- or premaxilla, lying below or in front of the 

 nose. This bone is well marked as a separate entity in all 

 the lemurs, monkeys, and apes. In the chimpanzee and orang 

 it unites with the maxilla sooner or later in the adult life of 

 these apes, and the suture between the bones is obliterated. 

 On the bony surface of the human palate a line is usually to 

 be found which was formerly taken as indicating a margin of 

 the premaxilla, but some authorities now consider it doubtful 

 if this line has any such significance. At all events one of the 

 earliest centres of ossification in the foetus appears at the sixth 

 week in the maxilla near the site of the future canine teeth, 

 and from this the portion of the bone which will carry the 

 incisor teeth is laid down during the following month apparently 

 by direct extension from this original centre. The details of 

 the formation of this part of the skull have perhaps not yet 

 been completely worked out, but it seems certain that all traces 

 of the premaxilla — if such ever existed apart from the maxilla — • 

 are very speedily lost. So absolutely unique is this disap- 

 pearance of the bone, that it makes the human embryo dis- 

 tinguishable from that of all other animals at a time when the 

 foot has the shape of the generalised reptilian type ; the five 

 digits are arranged like a fan and equally spaced apart, and 

 the cartilage of the little toe is quite as large as that of the 

 great toe.^ But according to the recapitulation theory, that 

 the embryological life of the individual gives a summary of 

 his ancestry, the disappearance of the premaxilla so very early 

 in fcetal Hfe would indicate {a) that the pre-human stock had 

 lost the bone, and therefore had already separated from the apes 

 at an immensely distant epoch ; or {b) that man has been able 

 in recent times to get rid of inherited structures which he did 

 not require ; or (c) that the common ancestor had a small 

 premaxilla which has become proportionately more prominent 

 in the apes and less so in man since the separation of the two 

 stocks. 



Light may be thrown on this problem by the parallel case 

 of the mammalian lower jaw. This bone is ossified in mem- 

 brane from one centre for each half, any little areas of cartilage 

 being probably the remains of Meckel's cartilage. In the 

 reptilian ancestors of the mammals each half " ought " to have 

 been composed of at least three bones. These, however, have 

 become completely fused and interlocked, so that no trace 

 1 " Digit " and " toe " are used to include the metatarsals. 



