History of infusoria, including the desmidiaceae and diatomaceae, British and foreign / by Andrew Pritchard.

  • Andrew Pritchard
Date:
1861
Catalogue details

Licence: Public Domain Mark

Credit: History of infusoria, including the desmidiaceae and diatomaceae, British and foreign / by Andrew Pritchard. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    144/1074 (page 124)
    50). These encysted spores resemble Chlamydomonads, and are called ' Pro- toccocoid' cells or globules, from their homology with the encysted cells of Protococcus. Occasionally, instead of one or several of the individual gonidia of a com- pound organism being encysted, the process ensues -with a gonidium deve- loped by fission into macrogonidia, and the whole mulbeny or Uvella-]ike mass becomes sm-rounded by a rigid envelope, either pretty closely applied, or sepai'ated by an interspace. Examples of the encysted condition will occur in the foUowrug account of the several groups of Phytozoa; it suifices at present to say that Prof. Wil- liamson and others have pretty clearly shown that Volvox aureus is only the encysted or ' still' form of V. globator, that Cohn has discovered the cysts of Steplianosphoira, Gonium, and Eudorina, and Henfrey those of Pandorina. The after-histoiy of the encysted spores of Phytozoa has not yet been elucidated: we have above referred to Cohn's researches upon it; but they are too indefinite to supply any positive information. The act of conjugation is common with many of the lower Algae, but has not been witnessed among the Phytozoa. Phases of Being aot) Alternation of Geneeation in Phytozoa.—From the preceding account of Phytozoa, it is evident that those best known exist imder a considerable variety of form—in other words, present several phases of existence, or, viewed in relation with a prevailing hypothesis, exhibit an alternation of generation. The whole history of any Phytozoon is com- prehended in the cycle of changes which the organism passes through; yet, under any transformation, it is the self-same being, and its existence may be said to extend from its most perfect through aU intermediate phases until the Hke degree of perfection is again attained. As happens in alternation of generations among other organized beings, the transition may not be direct and simple, but intermediate phases may reproduce themselves, and these again develope into other forms of existence, as accessory or collateral and usually imperfect cycles. Perhaps the metamoi-phoses in question are most striking in EugJence; for the contrast between the actively-moving, contractile, ever-changing being in one phase of existence, and the encysted, ' Protococcoid,' spore-like and motionless condition with a rigid unvarying outline, is so remarkable as to give colour to the hypothesis of the convertibility of animal into vegetable life, or of the transformation of animals into plants. It is not our intention at present to give illustrations of the varying phases ia the life of Phytozoa involved in the process of fission, or of a duplicative multiplication under its various forms. However, other more extended instances of transformation require to be noted, as observed by various microscopists,—although, it may be, some errors have crept in, from the difficulty of tracing the relation and succession of the different phases of being. As a very good example of the wide and varied range of existence enjoyed by most Phytozoa, we may adduce the Protococcus pluvialis (XIX. 20-31), of which the industry and perseverance of Prof. Cohn have obtained for us so complete an account. Accordmg to the researches of this eminent natii- raUst the simple plant in question, in its motile and still conditions, assumes the form and characters of many microscopic organisms presumed to be, and described by Ehrenberg and others as, distinct existences. To quote from Cohn's memoir (B. S. p. 559), Wo see that a single species oynng to its numerous modes of propagation, can pass through a number of very v^inous forms of development, which have been cither erroneously arranged as distinct genera, or, at least, as remaining stationary m those genera, altiiough, m