INTRODUCTION. 3 



latter in reference to animal organization, I here introduce an 

 abridgment of Schleiden's description of it. A delineation is 

 given in plate I, fig. 1, a, a, taken from the onion. This struc- 

 ture — named bv R. Brown, Areola or cell-nucleus, by Sclilciden, 

 Cvtoblast — varies in its outline between oval and circular, ac- 

 cording as the solid which it forms passes from the lenticular 

 into the perfectly spheroidal figure. Its colour is mostly yel- 

 lowish, sometimes, however, passing into an almost silvery 

 white ; and in consequence of its transparency, often scarcely 

 distinguishable. It is coloured by iodine, according to its 

 various modifications, from a pale yellow to the darkest brown. 

 Its size varies considerably, according to its age, and according 

 to the plants, and the different parts of a plant in which it is 

 found, from 0*0001 to 0-0022 Paris inch. Its internal struc- 

 ture is granular, without, however, the granules, of which it 

 consists, being very clearly distinct from each other. Its 

 consistence is very variable, from such a degree of softness as 

 that it almost dissolves in water, to a firmness which bears 

 a considerable pressure of the compressorium without altera- 

 tion of form. In addition to these peculiarities of the cvto- 

 blast, already made known by Brown and Meyen, Schleiden has 

 discovered in its interior a small corpuscle (see plate I, fig. 1, ftj 

 which, in the fully-developed cvtoblast, looks like a thick ring, 

 or a thick -walled hollow globule. It appears, however, to pre- 

 sent a different appearance in different cytoblasts. Sometimes 

 only the external sharply-defined circle of this ring can be dis- 

 tinguished, with a dark point in the centre, — occasionally, and 

 indeed most frequently, only a sharply circumscribed spot. In 

 other instances this spot is very small, and sometimes cannot 

 be recognized at all. As it will frequently be necessary to speak 

 of this body in the following treatise, I will for brevity's sake 

 name it the "nucleolus" (Kernkorperchen, "nucleus-corpuscle .") 

 I According to Schleiden, sometimes two, more rarely three, or, 

 as he has personally informed me, even four such nucleoli occur 

 in the cytoblast. Their size is very various, ranging from the 

 semi-diameter of the cytoblast to the most minute point. 



The following is Schleiden' s description of the origin of the 

 cells from the cytoblast. So soon as the cytoblasts have attained 

 their full size, a delicate transparent vesicle, the young cell, 

 rises upon their surface, and is placed upon the flat cvtoblast 



