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problem, are questions on which an analysis of the histogenesis may 
perhaps throw light. All three structures (i. e., epithelium, connective 
tissue (mesenchyme), and blood and blood-vessels) are certainly 
involved in such involution processes as occur in the closure of a gill- 
cleft. 
As an inevitable corollary of the above theses comes the state- 
ment that such structures as tonsils can be directly homologized with 
similar structures only within the narrow limits of a particular group 
of animal forms. If tonsil formation depends upon extrinsic morpho- 
genetic factors, it is clear that they are not themselves directly homo- 
logizable, but the processes upon which their development depends 
working in a homologous structural substratum. Attempts, such 
as have been or might be made, to homologize tonsils in birds, reptiles, 
and mammals, — not to include the amphibia, — are unjustifiable. 
There is no valid basis therefore for comparing the sublingual tonsils. 
of amphibia, for example, with the lingual tonsils of mammals. 
Finally, the ““ Kiemenreste’’ of MAURER require a word of comment 
from the view-point of the amphibian tonsil. MAURER has given the 
only detailed account of the development and structure of these 
organs. To review briefly his results: Three of them are developed: 
— dorsal, ventral, and middle “ Kiemenreste’’. The first of these in 
the frog persists for about a year and then disappears. The ventral 
one remains as a permanent organ in the frog (Rana), the middle one 
in the toad (Bufo). All three are developed in connection with the 
epithelium lining the branchial chamber, and arise as it becomes 
obliterated and disappears. The ventral body developes out of the 
cephalic portion of the branchial chamber which lies ventrally and 
medially. The epithelium of the branchial chamber becomes greatly 
thickened and at the same time numerous round cells appear in its. 
midst. In this way there is left after the disappearance of the branchial 
chamber and the gills, an ellipsoidal body attaining in the frog a length 
of 2 to 3 mm (GAuppP, 1899), and whose structure according to its. 
origin is a mixture of two elements, that were at one time epithelial 
cells and round cells, the latter predominating. 
It is clear that it is but necessary to conceive the persistance of 
the free surface, — lost in the disappearance of the branchial chamber, 
— to transform a ““ Kiemenrest,’’ — according to the mode of develop- 
ment given by MAURER, — into an amphibian tonsil. 
More recently the correctness of the interpretations of MAURER 
Anat. Anz. Bd. 42. Aufsätze. 39 
