GUINEA-PIG HAIR FOLLICLE FOR EXPERIMENTAL OBSERVATION 705 on 20 complete hairs from each of six intact guinea-pigs. It is evident that the daily growth rate is not constant. The hairs begin to grow at 0.22 + 0.01 mm day- •, gradually increase their growth rate to 0.61 + 0.01 mm day- • by day 22 and maintain this maximal rate for about five or six days. Towards the end of fibre growth the rate gradually falls. Duration of anagen The mean duration of anagen of the type 2 hairs, compiled from the individual mean growth periods of the guinea-pigs was 33.47 + 0.79 days. The mean definitive length of the hairs used to calculate the mean growth rate curve was also calculated. It was 15.51 +0.20 mm. Initiation of follicular activity In Fig. 4 are shown the percentages of hairs of each type commencing and finishing anagen in the dorsal region during each day of the period. HAIRS FIN. 20 ANAGEN {ø/0) 10 0 HAIRS COM, ANAGEN (ø/ø) o FIN. lO o COM. lo Figure 4. o FIN, I I I j - Type 1 hairs _ __ Typ, 3 hairs _ I,,l • I I I I I I I, ,I I 8 16 24 32 40 48 56 t AGE (days) Hairs Plucked Percentage of hairs commencing and finishing ariagert during the first 48 days of life. hairs __

70• JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Type 1 hairs Many hairs of this type finish their period of growth when the guinea- pigs are between 13 and 24 days old, with a peak in the number finishing growth at about 16 days. The definitive length of these hairs however pre- cludes the possibility that they had been produced entirely after birth. It is also apparent that follicles of this type enter anagen as early as day 13 and continue to do so until day 44, with the proportion becoming active reach- ing a peak at about day 32. Growth of these hairs is still in progress when they are plucked at 48 days. Type oe hairs Follicles of this type enter activity on every day from birth until 38 days, with a peak at about 14 days. Follicles finish their period of anagen from day 21 onwards, with a peak at about 43 days. As the duration of anagen of type 2 hairs was shown to be 33 days it is clear that some follicles must have become active before birth. Type 3 hairs No follicles of this type commenced anagen during the period studied. Follicles finish activity between 6 and 20 days, but with a marked peak at 15 days. Although the duration of anagen of type 3 hairs was not ascer- tained, their definitive length precludes the possibility that the hairs had been produced entirely after birth, and it is clear that once again the growth of this fibre type starts in utero. DISCUSSION It is quite clear that although there is no exact synchrony of follicles in a single region of the body, as in the pattern moult of other rodents, there is nevertheless a definite pattern of activity within each follicular type. However the follicles of one particular type are not in phase with those of any other type. While the present study confirms the finding of Bosse (11), that at birth and during the first 14 days of life all the hairs of the dorsal region are in anagen, it throws doubt upon the view that all synchrony breaks down after this time and that follicular activity takes on a mosaic pattern. The evidence of the present study, and from others at present in progress, suggests that there is a significant degree of synchrony within each fibre type over the first 150 days of life and possibly beyond. As yet we have not sufficient information to know whether the pattern of activity in the mid-side and ventral regions is similar to that in the dorsal.