8 THE VOYAGE OF H.M.S. CHALLENGER. 



In considering the process of reproduction, it is to be noted that the phenomena 

 recorded during a fortunate observation in one case only cannot be regarded as typical for 

 the entire group. It has, however, been perfectly established that reproduction can take 

 place by means of two conjugated frustules, or by the two valves of the same frustule 

 separating themselves so that the contents form a globular mass or sporangium, within 

 which the formation of one or more young frustules takes place. These young forms 

 sometimes exhibit duplication or fission, but this does not always happen, nor does it occur 

 in all frustules. On the contrary, some species give rise to very minute frustules which 

 in their turn form a new progeny. Thus when the endochrome occurs not in a more or 

 less amorphous condition but as numerous rounded masses of equal size and definite out- 

 line, the frustule is to be regarded as sporiferous, the rounded masses representing so many 

 sporules that are destined to reproduce the species. The escape of these sporules has been 

 observed by Eabenhorst in a species of Melosira, by O'Meara in a Pleurosigma, and by 

 myself in a Podosphenia, while Dr Cohn, as noted by Dr Pfitzer, saw a winged Amphi- 

 prora emitting small frustules from which the species could be recognised. 



It is further to be borne in mind that diatomaceous sporules will gradually develop 

 into the typical frustule either internally in the parent frustule, or externally in the spo- 

 rangium. Sometimes the parent frustule takes the place of the sporangium, in other 

 cases the sporangium is the production of the Diatom, destined for the incubation and 

 maturation of the sporules. 



These sporules are emitted in a free and solitary condition, but they are sometimes seen 

 in couples, as in Mastogloia. Here each couple is enclosed in an oval cyst, and all stages 

 of development, from two minute oblong greenish corpuscules up to large fully formed 

 frustules, can readily be traced. All the couples are immersed in a dense gelatinous 

 mass with which no extraneous bodies are mixed. Numerous round sporules have also 

 been observed to form a mass, and transitional stages have been traced from the round 

 green granular condition to that of spherical hyaline cysts which included a number 

 of small but distinct Naviculce. Among the cysts some were observed to move by 

 means of two very slender flagelliform threads, and these must be regarded as real 

 zygozoospores. 



It seems evident that the process of reproduction by means of sporules is the only one 

 that can explain the enormous rate of multiplication of the majority of the Diatomaceae, 

 while the other processes of duplication and reproduction will account for the greater 

 rarity of some forms. Viewed from a physiological standpoint, therefore, sporular repro- 

 duction is of the highest importance, as by this means countless individuals are formed, 

 all of which co-operate in supporting animal life by giving off the oxygen which is essen- 

 tial for the existence of the latter. 



