Mitosis in Proliferating Epithelium. 373 



becomes irregular, and ingrowth of epithelium occurs, comparable to, though 

 considerably less extensive than, that occurring in squamous epithelioma. If 

 the proliferation be set up in scar tissue, the resemblance to the latter 

 condition is striking. With the disappearance of the scharlach E the 

 process comes to an end and involution occurs, though the skin appendages 

 may be more or less destroyed. The character of the epithelial proliferation 

 is, therefore, intermediate between that of such affections as psoriasis and 

 lichen planus, in which epithelial hypertrophy without downgrowth takes 

 place, and epithelioma, in which downgrowth of epithelium is the character- 

 istic feature, and is the means by which neighbouring tissues are invaded and 

 extension of the growth occurs. 



When marked proliferation of the epithelium of the rabbit's ear has been 

 brought about under the conditions above described, sections hardened in 

 ^lemming's solution and stained by Heidenhain's iron alum hematoxylin 

 method show numerous mitoses, amounting occasionally to as many as five 

 mitoses to every hundred cells encountered. The mitotic cells are of large 

 size, being 10 /j, to 16 /z in diameter. In most cases the individual 

 chromosomes can be recognised to be more or less curved rods of varying 

 length, too closely-packed to be counted. In a relatively very small number 

 of the mitotic figures, however, it is possible to enumerate the chromosomes, 

 and in this way to determine the forms of mitosis present. 



The form most commonly met with appears to be the ordinary somatic 

 type shown in fig. 1,* which presents no essential points of difference 

 from the corresponding type in the testis of the rabbit, taken as a standard, 



<? ^ 



Fig. 1. —Normal Somatic Mitosis in Proliferating Epithelium of Rabbit's Ear. 



* The mitoses exhibited in this and the succeeding figures, all of which have the same 

 magnification (cp. scale in fig. 5), were drawn to scale in plan and elevation, and were then 

 modelled. The illustrations were prepared from the models. The use of models was 

 adopted in order to permit a simultaneous view of all the chromosomes present, in their 

 natural relation, instead of the succession of views at different levels afforded by the 

 microscope. 



