4 
Transactions of the Society. 
further contraction than Nos. 10 and 11, have secreted and become 
enveloped by a membranous sac, and have divided into two portions. 
No. 13 shows two of these young Biddulphias, the contents of 
which have become diffused over the whole area of the diatom, and 
are no longer concentrated in the centre. I have observed dozens of 
such cases in the Port Erin gathering. 
The next stage, which I have not thought it necessary to photo- 
graph, as not being very easily distinguishable from the fully 
developed frustule, is that of a delicate looking almost transparent 
frustule of the same form, and with similar spines to those of the 
fully developed Biddulphia. These were frequent in the gathering. 
Photographs Nos. 14 and 15 are those of frustules of Coscinodis- 
cus , the contents of which have been rejuvenised as in the case of 
Biddulphia. I have observed a large number of similar cases in the 
Port Erin gatherings, in all of which the valves of the young diatoms 
have been silicified and “ sculptured ” before leaving the parent. As 
to whether the young Coscinodiscus or the young Biddulphia in- 
creases in size after leaving the parent frustule, in either of the modes 
of reproduction dealt with in this paper, is no doubt a question of 
considerable difficulty ; but, judging from a series of photographs I 
have taken in illustration of the phenomenon of the formation of 
megafrustules after conjugation, I am inclined to the opinion that the 
increase in size of the newly formed diatom is attained by the last 
named process rather than by superficial growth, which my own 
observations lead me to think ceases after the young diatoms leave the 
protection of the parent cell. I do not think Mr. Murray’s experiment 
of treating the young diatoms by burning and nitric acid “ plainly 
warrants the conclusion ” that, not being sufficiently silicified to stand 
the process, their capacity for increase in size may be assumed. 
Photographs Nos. 16, 17, and 18 are those of Ditylum Bright - 
wellii, also found in the Port Erin gatherings. They appear to be 
clear cases of rejuvenescence of the cell and secretion of new frustules 
within the parent. 
Photographs Nos. 19 and 20 are those of frustules of Guinardia, 
which were not unfrequent in the Port Erin gathering, and are good 
illustrations of rejuvenescence. 
All the photographs of the marine diatoms, with the exception of 
Nos. 4, 6, 16, 17, and 18, show a magnification of 180 diameters. The 
remainder of the photographs, including all those of the fresh- water 
diatoms, show a magnification of 325 diameters. 
Sheffield : Sept. 1898. 
Addendum. 
Since writing the above paper, in a tube sent me from Port Erin, 
containing the results of a tow-netting made by Mr. Chadwick the 
previous day, and fixed by a weak solution of chromic acid, I have 
