48 GENERAL ANATOMY OP TIIE TISSUES. 



homogeneous, and subsequently exhibit a distinct membrane. In 

 jjathological formations, this character of the nucleus, which may be 

 called an undeveloped form, is very frequent, and the nucleus-like 

 structures in the Protozoa are also for the most part homogeneous 

 bodies. 



§ 9. Development of Cells. — With regard to the development of cells, 

 we have to distinguish between their free origin and their production 

 by the intermediation of other cells. In the former case the cells are 

 developed, independently of others, in a plastic fluid, the cytoblastema 

 of Schleiden, containing chiefly fat, protein, and salts in solution ; in 

 the other, or in cell-multiplication, the existent cells either produce the 

 so-called daughter or secondary cells within themselves, or multiply by 

 division ; eyidogenous cell formation and fissiparous cellformatioyi. Both 

 kinds of cell-formation agree in this, that the cell nuclei play a very 

 important part, and appear to be the proper centres of development of 

 the young cells. 



§ 10. Free cell-development is, in man and the higher animals, far 

 less common than has been hitherto assumed, and under this category 

 we can enumerate, so far as is at present known, only the development 

 of the chyle and lymph corpuscles, of the cells of certain glandular 

 secretions (spermatic cells, ova), and gland-like organs (closed follicles 

 of the intestine, lymph glands, splenic corpuscles and pulp, thymus) ; 

 lastly, of the cellular elements in the pregnant uterus, in the corpus 

 Fig. 2. liiteum, in the medulla of foetal bones, and in the soft 



„''-%. ..« ossifying blastemata. The separate steps of the process 

 '^^^^ ^ in this mode of cell-development have as yet been traced 

 ^^^m /mJ principally in the first-named cells, but much is yet want- 

 ci.3.9^ W^ ing to complete our knowledge of it. This much is cer- 

 tain, that the origin of the cells is always preceded by the development 

 of cell-nuclei, while it is doubtful, on the other hand, how these are 

 formed. In the chyle and in the spleen we see as the first indication of 

 cell-formation rounded homogeneous-looking corpuscles of 0*001--002 

 of a line, which, increasing somewhat in size, soon clearly appear 

 to be vesicles, and often, upon the addition of water, exhibit in their 

 interior, together with small granules, a large granule, like a nucleolus. 

 Whether this last, as is certainly the case in the dependent mode of 

 cell-development, arises before the nucleus and is the condition of the 

 development of the latter, or whether it is formed subsequently therein; 

 how, again, the nuclei themselves are developed, whether as originally 

 homogeneous corpuscles, which subsequently exhibit a difi"erentiation 



Fig. 2. — Contents of a Malpigliian corpuscle of the ox: a, small j b, larger cells; c, free 

 nuclei ; magnified 350 diameters. 



