NOTES ON SPECIES 289 



siliceous type phase populations are certainly a summer form of the far south, where no 

 wholesale change-over to the spineless chains takes place. In general the spineless chains 

 are very much less robust and less strongly siliceous than the hysirixjtype intermediates 

 from which they appear to develop in late summer farther north. Hendey's statement 

 (p. 329) '...In some specimens the bristles are entirely absent' should I think be 

 altered to ' In most specimens ..." to bring his description of the inerme phase into line 

 with our observations. 



I had already put forward the view that the change over to spineless chains might be 

 correlated with temporary shortage of silica, which would account for its complete 

 dominance over the hystrix/ type intermediates in some localities in late summer (Hart, 

 1934, p. 185). Analyses for silica were not then available, but subsequent work strongly 

 supports the suggestion, though it is possible that the seasonal change in temperature 

 may also be involved. The latter, however, is very slight in the regions with which we 

 are concerned, less than 3° C. between the peak of the main increase and the time of 

 maximum development of the inerme phase. It may be mentioned that in fresh material 

 the chains are often extremely long — up to 2 mm. 



I have never seen gelatinous colonies of Corethron such as Hendey (1937, p. 327) 

 describes, but the exceptionally small and weak far southern type of Corethron is often 

 associated with Phaeocystis in pack-ice and develops with that organism in adjacent 

 waters. From Hendey's description of the pale-staining mucilaginous groundwork, 

 with deeply staining granules in addition to the Corethron cells, it seems probable that 

 he was looking at a mixture of the two distinct organisms. Where it is abundant, 

 Phaeocystis jelly always tends to entangle everything else in the samples. That the 

 granules could be microspores appears very doubtful. Gross (1937, p. 39) doubts 

 whether microspores really exist among centricate diatoms. I have seen inclusion 

 bodies similar to those described by Karsten (1905, pp. 108-9, Taf. XIV) as microspores 

 of Corethron, and mentioned by Hendey, but always in individuals considerably larger 

 than the small weak ice-edge phase. These bodies might indeed give rise to the latter — 

 they are often nearly as big while still within the mother-cell — but are they really 

 microspores? 



It is noteworthy that in a large population of the small weak ice-edge Corethron one 

 may at first find no large individuals, but if the stations are closely spaced one soon finds 

 a small proportion of large individuals produced by recent auxospore formation. On 

 occasions the proportion of large individuals was clearly increasing with time, and the 

 auxospore formation could be seen in progress. 



It appears to me, therefore, so far as we can say at present, that the real order of 

 events is something like this : Far south minute (' type phase ') Corethron and Phaeo- 

 cystis subsist together in the pack-ice. Both forms multiply rapidly when liberated in 

 summer, but the Phaeocystis soon decreases. Some of the Corethron cells, already near 

 the lower size limit for the species, soon begin to form auxospores. From the large 

 cells so developed the small-celled population is maintained— perhaps merely by the 

 well-known progressive diminution through continued division, but quite probably 



