3i8 



DISCOVERY REPORTS 



which may be mentioned here though they are quite separate from the main mass of 

 grooves. These are to be seen in Plate XXX, figs, i and 3. The four rough sketches in 

 Fig. 51 show some typical examples and give an idea of the kind of variations which 

 occur. 



The best method of estimating the number of the main ventral grooves is to count 

 their anterior terminations, beginning at the middle of the chin and counting along the 

 mandible past the angle of the gape up to the last groove near the eye, and doubling 

 the result. 



The estimation of the number of grooves is of no particular value except (as in 

 the case of other characters) for the purpose of fixing the normal condition and 

 range of variation of this character for purposes of general comparison. Differences 

 which occur in the numbers of ventral grooves of whales from South Georgia are 

 evidently due simply to individual variation, for the diff'erences between parent and 

 foetus (in cases where the ventral grooves of the foetus are fully developed) are of the 

 same order as the differences between adults. Whale No. 54 for instance had 94 grooves 

 and its foetus had 78, while whale No. 154 had 80 grooves and its foetus 96. 



The variations in the numbers of grooves is best shown by the following table, in which 

 the records are analysed on the same principle as the table for numbers of baleen plates 

 given on p. 316. 



Blue Whales 



An inspection of the above table shows again that no distinction exists between 

 the sexes, and the fact that the averages for South African whales are slightly lower 

 than for South Georgia is more likely due to the small number of records than to the 

 existence of any real distinction. 



The ventral grooves appear in the foetus earlier than the baleen plates. The first 

 traces are to be found when the foetus measures from about i-o to 1-25 m. The anterior 

 ends appear to materialize first and then spread backwards, and by the time the foetus 

 reaches 2-0 m. they are usually sufficiently complete and well defined to be counted. 



