288 THE MICROSCOPE. 



an elongation of the membranous sac appear, giving a 

 semi-lunar sort of chamber ; this, as the endochrome 

 elongates, becomes more defined, until it has the form and 

 outline of the chamber at the perfect extremity. The 

 obtuse end — b of the frond is at the same time elongat- 

 ing and contracting ; these processes go on ; in about five 

 hours from the division of the one segment from the other, 

 the appearance of each half is that of a nearly perfect 

 specimen, the chamber at the new end is complete, the 

 globular circulation exterior to it becomes affected by the cir- 

 culation from within the said chamber ; and, in a few hours 

 more, some of the free bodies descend, become exposed to, 

 and tossed about in the eddies of the chamber, and the 

 frond, under a l-6th power, shows itself in all its beau- 

 tiful construction. E is a diagram of one end of a C. didy- 

 motocum, in which the same process was noticed. 



The Euastrum Didelta is well worthy 

 of attention, as well as many other species, 

 the Xanthidium Penium, Docidium, &c. 



The Arthrodesmus Incus has a very 

 beautiful hyaline membrane stretching 

 from point to point, cut at the edges, 

 Flg ' ' something like the Micrasterias. This is 



represented at fig. 157. 



The Mode of Finding and Taking Desmidiacece. — As 

 the difficulty of obtaining specimens is very great, it will 

 materially assist the efforts of the microscopist to know 

 the method adopted by Mr. Ralfs, Mr. Jenner, and Mr. 

 Thwaites. " In the water the filamentous species resemble 

 the Zygnemata ; but their green colour is generally paler 

 and more opaque. When they are much diffused in the 

 water, take a piece of linen, about the size of a pocket 

 handkerchief, lay it on the ground in the form of a bag, 

 and then, by the aid of a tin box, scoop up the water 

 and strain it through the bag, repeating the process as 

 often as may be required. The larger species, Euastrum, 

 Micrasterias, Closterium, &c, are generally situated at the 

 bottom of the pool, either spread out as a thin gelatinous 

 stratum, or collected into finger-like tufts. If the finger 

 be gently passed beneath them, they will rise to the sur- 

 face in little masses, and with care may be removed and 



