AUTHOR’S ABSTRACT OF THIS PAPER ISSUED 
BY THE BIBLIOGRAPHIC SERVICE, JANUARY 6 
FACTORS INVOLVED IN THE FORMATION OF THE 
FILUM TERMINALE 
GEORGE L. STREETER 
From the Department of Embryology, Carnegie Institution of Washington, 
Baltimore, Maryland 
THREE TEXT FIGURES 
In a study recently published by the writer! on the develop- 
ment of the cartilaginous capsule of the ear in human embryos 
it was pointed out that the changes in size and form which the 
capsule undergoes during its development are accomplished not 
only by a progressive differentiation, but also in part by a retro- 
gressive differentiation of its constituent tissues. The margins 
of the cartilaginous cavities are in a continual state of change; 
they exhibit an unstable equilibrium between two opposing ten- 
dencies: on one hand, toward the deposit of new cartilage, and 
on the other, toward the excavation of the old. The margins 
thereby are always advancing or receding, and as a result of 
this there is provided a suitable suite of chambers for the 
contained membranous labyrinth in all stages of its development. 
It is the feature of retrogressive differentiation or dedifferen- 
tiation that I wish particularly to recall here. The fact that 
certain areas of cartilaginous tissue revert to an earlier em- 
bryonic type and are subsequently redifferentiated into a tissue 
of a widely different histological character, is very clearly shown 
in the case of the otic capsule, and is a factor of great embryo- 
logical significance. Such a process of retrogressive change, 
combined with redifferentiation of the same tissue, greatly in- 
creases the facilities for and the range of certain structural 
1 Streeter, G. L., 1917. The factors involved in the excavation of the cavities 
in the cartilaginous capsule of the ear in the human embryo. Amer. Jour. Anat., 
vol. 22. 
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