the gills which takes place to such a marked degree during develop- 
ment might without study be thought to indicate that the adult 
thyroid occupies a different position from that in which it originates. 
This is not actually the case as it develops entirely in situ and never 
moves from its original position, although in its relation to the hyoid 
arch and head organs the adult thyroid is transferred much further 
tailward than was the anlage. This apparent change of position is 
clearly due to the same cause which makes the entire gill area seem 
Fig. 3. A series of sections from various parts of the gill region showing the ven- 
tral portion of the pharynx and the thyroid anlage in outline, the thyroid portion is 
stippled. About 85 diameters. A. Through posterior part of first gill. 2. Anterior 
part of second gill. ©. Posterior region of second gill. D. Posterior of third gill. E. 
Through fourth gill. #. Through fifth gill, @. Through seventh gill. 4. Through 
ninth gill. J. Outline of the embryo showing gill region the thyroid area being stippled, 
with the above sections indicated by corresponding letters. 
to move caudally a process which as stated above will be fully 
described later. The remarkable individual variation among these 
animals must always be remembered in the study of any one of 
their organs, and in the development of the head region I have ob- 
served many minor variations. 
KUPFFER !) in his study of Bdellostoma failed to find the thyroid 
as most of his embryos were too young to show it and much of his 
material was in a very inferior state of preservation. Yet in his 
1) Studien zur vergleichenden Entwickelungsgeschichte des Kopfes 
der Kranioten. Heft 4. Zur Kopfentwickelung von Bdellostoma. 
München u. Leipzig, Lehmann, 1900. 
