92 MANUAL OF HISTOLOGY. 



eluding a still larger number of pus-corpuscles, and altered in their shape 

 on. that account (d). 



On being liberated from the cells in which they have been contained, 

 these structures display all the characters of pus-corpuscles (e). But even 

 in ciliated cells, such as are to be met with in the mucous membrane of the 

 respiratory organs, these pus-cells may be found (/), as also in the flattened 

 epithelium of the bladder (g). Such things are, however, capable of 

 another explanation. Pus-corpuscles, elements whose vital contractility 

 is beyond a doubt ( 49), may have wandered into the epithelial cells 

 from without. Such an immigration of strange elements has been proved 

 to take place recently into the cells of some morbid growths (Steudener). 



The same appearances are presented farther by psorospermia, peculiarly 

 puzzling single-celled structures, which are frequently to be found, in the 

 bile-ducts and intestinal canal of rabbits, and which are looked upon as 

 parasitic organisms (fig. 92). 



57. 



Of the various modes of reproduction or proliferation of animal cells, 

 that which goes by the name of endogenous growth has been long known, 

 although its details have been variously interpreted. But it is only com- 

 paratively recently that segmentation has been generally recognised, and 

 mainly so through the numerous proofs adduced by two observers, Remak 

 and Virchow the first from the wide field of embryology, the latter from 

 pathology. From them emanated a contradiction of a doctrine put forward 

 by Schwann, which influenced for a long time all our views in regard to 

 histogenesis ; and the opposition soon became so widely supported as com- 

 pletely to throw Schwann' s theory into the shade. 



According to the latter, animal cells are formed free, that is, indepen- 

 dently of any previously existing. " There is," says he, " either in those 

 cells already present or between them, a structureless substance, the cell- 

 contents or intercellular substance ; this matter (cytoblastema) possesses in 

 itself, to a greater or less extent, according to its chemical constitution and 

 degree of vitality, the power of giving rise to new cells. The genesis 

 of cells represents in organic nature what crystallization does in inorganic." 



In the first place, says Schwann, there springs up in this cyto- 

 blastema a minute corpuscle, namely, the nucleolus, and owing to the 

 attraction of this body for surrounding molecules of matter, layers of these 

 are precipitated, as it were, around it, giving rise to the nucleus. By a 

 repetition of the process, a second layer is deposited upon the latter, 

 which, though differing from the surrounding medium, is not yet sharply 

 defined, but becomes so later. This deposit, hardening externally, forms 

 the substance and wall of the cell. At the commencement the newly 

 formed envelope lies closely round the nucleus the cavity of the cell, 

 and with it the entire structure, being still small, but the wall increases 

 subsequently in size, and the whole obtains finally its specific contents. 



To this was added later another theory, according to which the nucleus 

 of certain cells is enclosed, in the first instance, by the future specific 

 contents, and then only, around this mass, containing in its interior 

 the nucleus (the so-called "enveloping sphere"), is formed a wall by 

 solidification of part of the deposit, which brings the whole structure to 

 completion. 



For many years these two modes of origin of the animal cell appeared 

 to be proved beyond all doubt, and the sole differences of opinion which 



