92 ARCHES AND DIXON, OX DESMIDIACEJE. 



been absorbed. I hazard such a conjecture the more, because 

 something very like this seems actually to occur iu some 

 species of Gloeocapsa, or allied forms, in which the concentric 

 gelatinous layers seem to harden into so many frangible 

 investments. 



In the foregoing very brief account of the mode of cell- 

 division which occurs in this family, it will be noticed that I 

 look upon the formation of a septum at the isthmus as the 

 preliminary or initial step in the process. Now it appears to 

 me that the accompanying figures represent individuals, 

 which, having taken on them the vegetative growth, or effort 

 to repeat themselves by transverse division, through some 

 inexplicable cause have omitted the formation of a septum. This 

 not having taken place simultaneously with the vegetative 

 activity being aroused, which, being in full energy, the frond 

 in each case proceeded to the development of new growth) 

 which, according to the law which prevails in this family, took 

 place, as usual, between the older segments. In consequence, 

 therefore, the resulting formation consisted of but one cavity, 

 and fresli endochrome being added, they each became entirely 

 filled. Nor do I think some instances lately under obser- 

 vation, and to which I will just allude, are a contradiction to 

 this. Specimens of Penium cylindrus, to all appearance per- 

 fectly healthy, and manifestly undergoing growth, lately 

 occurred to me ; of these, a few individuals presented them- 

 selves, in which various stages of the new growth, produced 

 in the usual manner in this species on a line with the older 

 segments, had been accomplished, in some cases the fronds 

 haying attained to double the ordinary length — but in none 

 of the instances referred to was any appearance of a septum 

 evident. That the fronds had added to their length by recent 

 new growth was proved by its usual colourless cell-wall, as 

 compared with the red-tinted older segments. That there 

 was no septum was proved by the granular particles partaking 

 of a circulatory motion at and past the central point ; and 

 indeed, when present, it is readily seen as a transverse line. 

 No further alteration took place in these specimens kept for 

 some time on a slide. Notwithstanding their not very 

 unusual appearance, for the absence of the central septum was 

 not very striking, and might not be noticed at first sight, 

 unless cloSely looked into, as well as there being no external 

 aberration in form, it appears to me that the specimens of 

 Penium cylindrus alluded to were so many illustrations of the 

 same abnormal mode of growth, extreme cast's of which 1 

 baVe tried to depict in the drawings. For 1 cannot easily 

 understand, without the original separation of the primordial 



