164 Mr. H. J. Carter on the Conjugations of 



Neither the endochrome nor the nucleus appears to be com- 

 pletely reformed and defined until the frustule has undergone 

 one or two duplicative divisions. 



Precisely the same number of coverings accompanies the 

 sporing of Spirogyra, and especially of StaurocarpuSj where the 

 spore is formed between the two conjugating cells, as in that of 

 Navicula serians, viz. : — (1) the old cells of the -filament, which 

 correspond to the frustules; (2) the newly secreted part of 

 Spirogyra or cell of Staurocarpus, into which the contents of 

 both the parent cells are poured confusedly, corresponding to 

 the gelatinous envelope; (3) the thick brown capsule formed 

 round the latter, which answers to the capsule of the spore of 

 Diatomese; (4) a thin transparent cell within this, which is 

 analogous to the sheath of the sporangial frustule; (5) and, 

 lastly, the young Spirogyra itself, which is homologous with the 

 sporangial frustule, and which, on germinating, bursts forth with 

 the bands of chlorophyll of the species nearly all restored to their 

 proper form and position. But there is this difference, viz. that 

 the spore of Spirogyra, &c., rests from its labours for a certain 

 time before the young sporangial plant comes forth, while that 

 of the Diatomese continues its progress throughout the process 

 uninterruptedly, until the sporangial frustules are eliminated in 

 full activity. 



The efi'ect of the conjugation, then, in Navicula serians is to 

 produce two sporangial frustules nearly twice as large as the 

 two conjugating ones — an increase in size which may be inferred 

 to owe its origin to the contents of each of the conjugating 

 frustules having become sufficient in quantity for duplicative 

 division just before they entered into conjugation, whereby they 

 would produce two sporangial frustules nearly twice as large as 

 their ordinary size. 



Sometimes the process may be wholly abortive, from accidental 

 circumstances ; at others it may be only partially so, and thus 

 produce only one sporangium capable of developing a frustule, 

 while the other remains sterile for want of sufficient material to 

 work upon, as in PI. IV. figs. 4 & 15. 



The average largest size of Navicula serians present with the 

 conjugations was that of fig. 9, of which the measurement is 

 above given, and that of the sheath within which it is enclosed 

 a little larger. 



When these conjugations are slightly boiled in nitric acid, the 

 sheaths remain entire, as stated by Dr. Griffith ; but after pro- 

 longed boiling, I could not recognize any trace of them. Pos- 

 sibly they may have escaped my observation, as the same natu- 

 ralis thas stated [apud Pritchard) that those of Navicula amphi- 

 rhynchus resisted the action of a " red heat '^ as well. 



