1094 
DB,. H. WATNEY ON THE MINUTE ANATOMY OP THE THYMUS. 
there are all transitional forms between flattened epithelioid cells and ciliated epithelium 
(see the smaller left-hand cyst in Plate 94, fig. 89). 
It is quite easy to show gradual transitions, from small concentric corpuscles to the 
large cysts containing columnar ciliated epithelium. The various steps may be artificially 
classed as follows : First, concentric corpuscles, the peripheral cells of which are con¬ 
nected to, and continuous with, the network of the medulla (see Plate 94, fig. 85) ; 
secondly, concentric corpuscles, containing in their centre a mass of degenerated cells, 
which have been separated from the surrounding cells by a process of vacuolation (see 
Plate 94, fig. 86); thirdly, small cysts which are bordered by epithelioid cells, and 
which contain either a degenerated cell mass or a mass of haemoglobin (see Plate 94, 
%• 87); fourthly, cysts partly lined by epithelioid cells, and partly by ciliated epithe¬ 
lium (see Plate 94, figs. 88 and 89); and fifthly, large cysts, in some cases of nearly 
half the width of the follicle, lined by columnar ciliated cells, and generally containing 
masses of haemoglobin in their interior (see Plate 94, fig. 84). 
The ciliated cysts can therefore be shown to arise from concentric corpuscles ; and 
these, as we have seen above, always primarily from connective-tissue-corpuscles. We 
therefore come to the conclusion that the connective-tissue-corpuscles can undergo 
certain changes , which finally transform them into ciliated epithelium. There is no 
doubt a considerable d priori improbability in this statement, as we should expect 
ciliated epithelium to be found in association with a higher organization, and not as one 
of the methods of involution or degeneration of the tissues; and further, the view 
stated above is incompatible with those which have been so long held concerning the 
respective functions of mesoblast and hypoblast. There seems, however, no other con¬ 
clusion left to us, unless we grant that in the development of the thymus, certain traces 
of epiblastic tissue have been left: a view which we have discussed somewhat at 
pages 1091 and 1092, and which, as we shall afterwards see in studying the develop¬ 
ment, is not established.* 
Changes in the Thymus during involution. 
Some of the changes occurring in the thymus have been mentioned at pages 1079 and 
1080; others, of which we have to treat, are: the formation of connective and fibrous- 
tissue in the follicle ; the increase of the interfollicular connective-tissue, the invasion of 
this tissue into the follicle, the deposition of fat in this invading tissue and between the 
* Ciliated cells have long been known to occur on the surface of the peritoneal cavity of Erogs. 
Waldeyer (91) considered them only a continuation of the genital mucous membrane. Klein (92) 
described ciliated endothelium in the peritoneal cavity of Erogs, and figured (see his fig. 18) a vacuo¬ 
lated cell, containing fine cilia projecting into the vacuole. Neumann (93) noticed a transition between the 
endothelium of the peritoneal cavity of the Erog and ciliated epithelium; and Nicolsky (94) found that in 
all Frogs ciliated epithelium exists on the serous cavities, and in young male Erogs on the pericardium. 
This ciliated epithelium is not to be classed as belonging to the female genital organs, but as transformed 
endothelial cells. 
