212 DR. COHN, ON THE DEVELOPMENT OF 



latter vary in number, from one to any number in the same 

 envelope, being single, for instance, in Chlami/dococciis {Pivto- 

 coccus) and Cklamydomonas, eight, as in Stephanosph/era, or 

 innumerable, as in Volvox. In either case, the primordial 

 cells present the character of simple primordial sacs, which 

 are not immediately surrounded by any rigid cellulose mem- 

 brane ; consisting merely of a fine-grained protoplasm, co- 

 loured red or green by chlorophyll, or a peculiar oil, and often 

 prolonged into mucoid filaments. The primordial cells 

 themselves are produced peripherally into a colourless point, 

 from which arise two vibratile filaments, which penetrate the 

 investing sheath through two openings, and pi'oject into the 

 surrounding water. The reproduction is effected by the divi- 

 sion of all, or, as in the case of Volvox, of some of the pri- 

 mordial cells, which, in this case, come eventually to resemble 

 the parent organism ; or the contents of the germinating 

 primordial cell are more minutely subdivided into motile 

 zoospores of much smaller size (termed microgonidia\ whose 

 ultimate destination is unknown ; the other or larger form are 

 termed macrogonidia. The latter, lastly, may assume the 

 ' quiescent ' state, each primordial cell within the delicate 

 ' investing membrane' secreting around itself a second, more 

 dense cellulose membrane, which is not perforated by the 

 motile cilia, but is in close opposition with the primordial cell, 

 just as in the common plant-cell the cellulose membrane 

 encloses the primordial sac or utricle. In this^ manifestly 

 vegetative, protococcus-\\ke condition, the cells may remain 

 toi-pid and without any movement for almost any length of 

 time, until iheir dormant energies are re-awakened by the 

 addition of water. It would even appear that in many cases, 

 a previous desiccation is required, to render these winter- 

 spores capable of germination. (These winter-spores, in dif- 

 ferent states in Volvox, have been supposed by Mr. Busk to 

 be represented in the forms termed V. aurejis, and V, stellatus.) 

 In speaking of Gonium, a genus referred by Cohn to the 

 Volvociiia, he remarks, that his own observations have led him 

 to perceive that its structure is somewhat more complex than 

 it is usually described as being. 



In the first place, the entire tablet is surrounded by a 

 perfectly colourless, transparent envelope, presenting the form 

 of a flattened spheroid, the axis of rotation being shorter than 

 the other two. This envelope is not surrounded by any firm 

 or cellulose coat, and appears to be simply gelatinous or 

 mucoid ; and it is, consequently, not very readily visible. 

 As in other similar cases, it is best brought into view by the 

 addition of some colouring matter to the water. The green 

 globules are simple or primordial cells — properly of an 



