70 ESCHRICHT AND REINHARDT 



the soft cells of the smooth intervals is not so sharp^ but that the nearest approaching cells of the 

 latter may also become somewhat horny. 



We have stated before that the formation of the cells on the whole surface of the pulp takes 

 place continually. But in this continuous formation of cells certain periodical changes can very 

 easily be traced, especially by those small transverse elevated lines which are seen more or less 

 distinctly on the two lateral surfaces of each blade of whalebone at pretty regular distances from 

 each other. John Hunter pointed out^ that these transverse rings are imperceptibly lost in the 

 extremely fine membranes that divide the layers of the soft intermediate substance (gums) from 

 each other, though, it is true, only rather indistinctly. Besides this, he observed in each trans- 

 verse ring an immediate connection between one of the extremely fine membranes of the soft 

 intermediate substance and the most external layer of the cortical tissue beneath the ring. But in 

 his supposition that he might conclude from this that the cortical tissue of the whalebone-blades 

 is formed out of the soft intermediate substance, we by no means agree. To explain the connection 

 between the two we need only, in addition to the description we have just given of the cells that 

 are, and those that are not, converted into horn, think that the periodical changes in the formation 

 of both are contemporaneous, and then it will easily be seen that the relation between them must 

 be exactly as Hunter has already described it to be. All the layers of the cortical part can only 

 be supposed to be formed from the surface of the pulp-blade. The more rapidly the blade grows, 

 especially when the animal is young, the fewer layers will be added from within to those layers 

 of the enamel that already have become horny, and the thinner this enamel will be found to be ; 

 but the more slowly the growth of the blade takes place, as in a more advanced time of life, 

 the thicker the enamel will be. In the subsidiary blades of the twenty-two feet long individual 

 there is a peculiarity which leads us to suppose that the formation of the cortical tissue may even 

 be quite suspended for a time. For throughout the whole length of the set of whalebone, except 

 a small portion anteriorly, these small ribbon-hke blades want all the cortical covering in the middle 

 third of that length, by which they project from the soft and white intermediate substance, and in 

 this part they appear hke mere fasciculi of bristles, completely resembling those on the free 

 edge of the blade, whereas the cortical layer is formed perfectly normally, not only above but also 

 beneath the place mentioned. 



The rings of the whalebone-blades with the fine membranes often attached, even in dried 

 specimens, and having always their free margins turned towards the palate, prove as we have said 

 a periodical change in the formation of the cortical part. But we are not yet in possession of the 

 necessary data for ascertaining the length of these periods. We are able, however, to state that 

 they do not correspond to the revolutions of the years. Por in the blades of our twenty-two 

 feet long skeleton we find already many such rings, and yet it is certain that, considering the 

 rapid orowth of these animals, the Greenland whale, which at its birth is about thirteen feet long, 

 must be supposed to have attained a size of twenty-two feet by the end of the two ensuing years. 

 We should be far more inclined to explain the interruption of the cortical tissue in the subsidiary 

 blades of the same individual just mentioned as evidence of a periodical change in the cortical 

 formation corresponding to the change of the seasons of year, as it would denote an age of two 

 years, just the age which this half-grown animal may be supposed to have attained, judging by 

 its size. 



^ ' Philsophical Transactions/ vol. Ixxvii (1787), p. 404. 



