300 DISCOVERY REPORTS 



Before attempting to give an explanation of the coarse laminations of the core of the plug and their 

 apparent absence from the outer covering, it is necessary to draw attention to certain facts about the 

 rate of growth of the parts of the skull involved in the formation of the auditory region. In the feature 

 most directly concerned, the tympanic bulla (and consequently the tympanic annulus), there is no 

 easily measurable difference in size between that of the newly born juvenile and the most fully grown 

 adult. In view of its association with the tympanic annulus and its importance in regulating the tension 

 of the tympanic ligament, it may be assumed that the glove-finger also remains constant in size through- 

 out the life of the animal. It may further be assumed that the diameter of the bony external meatus, 

 which is directly determined by that of the glove-finger, undergoes no enlargement. Measurement of 

 the mesial distance between the two tympanic bullae in skulls of various ages shows that here too the 

 increase in dimension is very slight. The characteristically great lateral growth of the posterior region 

 of the skull in the Mysticeti is apparently almost wholly associated with the lateral extension of the 

 zygomatic process of the squamosal, the paroccipital process and the mastoid process of the tympano- 

 periotic. During the lateral extension of the above-mentioned bones which are concerned in the forma- 

 tion of the bony external meatus, the wax plug would tend to be drawn away from its contact with the 

 glove-finger, the interval so formed being simultaneously filled up by active formation of squamous 

 epithelium from the former's zona germinativa. The rapidity with which this tissue develops can be 

 judged from the great amount of imperfectly keratinized epithelium in the core of the plug. It would 

 appear that there are regular periods during which lateral extension is either quiescent or very slow 

 and during which a thin continuous layer of fully keratinized epithelium covered externally by ceru- 

 men is formed over the end of the glove-finger. The ceruminous layers correspond in position with the 

 thin, dark -coloured bands which are to be seen on the cut face of the plug. The complete reversal of cur- 

 vature of the epithelium of the second lamina could be explained by the streaming of lipoids between the 

 tissue columns during the quiescent period (see diagrams, PI. XVI, figs. 3 and 4). The greater thickness 

 of the more distal laminations of the core of the plug is consistent with the greater rate of growth of the 

 juvenile individuals, and although there is a certain degree of irregularity in the amount of tissue formed 

 during each active period, the thickness of the laminations decreases exponentially over the greater part 

 of the series.* In the graph, Text-fig. 2, the length of the core of the plug at each quiescent period is 

 plotted against the lamination number beginning at the distal end. The last two laminations are un- 

 compacted so that a sharp rise appears at the end of the curve. This diminution of compactness must 

 influence the thickness of the laminations near the proximal end of the core, which probably accounts 

 for the fact that this part of the series is not exponential. Since the measurements were obtained from 

 a photographic enlargement (xio) of the natural size, a small allowance must also be made for 

 spherical aberration at the extremities of the series. If the end of the first lamination corresponds with 

 the time of birth of the animal, the body length would be 6-5 m. (Mackintosh and Wheeler, 1929) and 

 12 m. at the end of the second lamination, which would represent the nursing period. The third lami- 

 nation, which is short but very conspicuous, could represent the free feeding period during the animal's 

 first polar season. 



If each subsequent quiescent period (the interlamina space) represents the period of migration 

 north or south, the laminae proximal to the first three would represent five- to six-monthly periods and 

 sexual maturity would occur during the formation of the sixth lamination. The estimated length at 

 sexual maturity in male specimens is 19-5 m., and if this measurement is correlated with those of im- 

 mature specimens and that of the specimen obtained during 1954, a male of 20-7 m. possessing twelve 



* The implications of the relation of laminae to physical maturity have not been overlooked. When this is attained it might 

 be expected that increase of the meatal length would cease, but it seems likely that in Cetaceans as in other mammals post- 

 physical maturity increments in extra-cranial processes, involving lateral expansion of the skull as a whole, can continue. 



