50 THE CELL DOCTRINE. 



in the many instances in which authors on 'cells' 

 have described and figured more than one nucleolus 

 in a nucleus, there has been either an incipient divi- 

 sion of the nucleus into discs, or the nucleus has 

 consisted of two or more discs ; the nucleoli of those 

 authors having been the minute and highly refract- 

 ing cavities or depressions in the discs. If this has 

 really been the case, it afl:brd8 additional evidence, I 

 think, that the reproduction of cells by the process I have 

 described — namely, division of the nucleus of the parent 

 cell — is universal — so numerous have been the in- 

 stances in question. I may refer to the figures given 

 by Schwann, who examined nearly every tissue, and 

 to those of Schleiden, whose observations have been 

 so extensive on plants. I think, indeed, that many 

 of the figures of Schwann aftbrd evidence of the 

 division in question having taken place. It is to be 

 recognized in his delineation of the cells of cartilage, 

 cellular tissue, middle coat of the aorta, muscle, ten- 

 don, feather, etc. The same remark is applicable to 

 a figure given by Reichert of ciliated epithelium 

 cells. Dr. Henle found that in the layers of his 

 ' pflaster-epithelium ' cells, the nucleus, very distinct 

 in the lower cells, had almost disappeared in those 

 situated in the upper part. From this observation, 

 and from the presence of two nucleoli in some of the 

 nuclei figured by this observer, as well as from the 

 nucleus becoming more granular,! think it extremely 

 probable that these cells (including those of the epi- 

 dermis), are reproduced by the process just referred 

 to,— division of the nucleus ; additions being no doubt 

 continuously made at the lower part of the layer, by 

 ' which cells previously there are pushed farther out.' 



