92 
„Der ganzen Lage dieser Gebilde nach kann es sich wohl nur um 
ein Derivat der Kiemenspalten oder des Kiemendarmes selbst handeln. 
Zieht'man das Aussehen und den feineren Bau der Drüsenblasen in 
Betracht, so würde ihre Aehnlichkeit mit Schilddrüsenblasen für die 
Deutung des Organes als Gl. thyreoidea sprechen.“ 
„Zu diesen Ausführungen, die, soweit sie in Anführungszeichen 
stehen, vor 9 Jahren niedergeschrieben worden sind, sei bemerkt, daß 
ich heute nicht daran zweifle, daß die geschilderten Drüsenblasen der 
Thyreoidea und nicht etwa einer Thymus angehören, da es sich doch 
um eine rein mediane, unpaarige Drüsenbildung handelt u. s. w.“ 
This article appeared just at the time when I was engaged in a 
study of the thyroid in embryos of Bdellostoma and I am now able to 
furnish the developmental evidence which SCHAFFER states is essen- 
tially necessary for a definite interpretation of these scattered inter- 
branchial Drüsenblasen. 
I am entirely indebted to the generosity of Professor BasHrFoRD 
Dean for the very rare and valuable material on which these obser- 
vations were made. My thanks are also due Prof. Dean for many kind 
suggestions during the work. 
Development of the Thyroid. 
The youngest embryo in which the thyroid anlage was found to 
appear may be briefly characterized thus. The mouth and nasal canals 
are directed ventrally and more caudad in position than when fully 
developed. An embryo of about this stage is shown in Deran’s?) 
plate XIX, fig. 61 and plate XXV, fig. 126. The gills have “shifted” 
a considerable distance back of their original position so that a long 
interval now exists between the hyoid cleft and the first branchial gill, 
this “shifting” will be discussed at length in a subsequent paper on 
the head development of Bdellostoma. The gill structures are also 
completely drawn in to their adult position being no longer laterally 
spread out as in younger stages, Dman’s plates XXII to XXIV. The 
first few anterior gills are beginning to form the pouches although the 
posterior ones are still tubular in structure. At this stage of deve- 
lopment the thyroid anlage is clearly shown as a median down-pushing 
from the ventral floor of the pharynx throughout the entire gill area. 
Thus a long trough is formed the walls of which are closely approxi- 
mated so that a space between them exists only in a few regions, and 
the narrowness of the trough is easily explained by the general laterally 
compressed condition of the gut in the gill area, its floor here being 
very narrow indeed. Fig. 1 shows an outline camera drawing of a 
1) On the Embryology of Bdellostoma Srourı. Festschr. zum 70. Ge- 
burtstage von C. v. Kurrrer. Jena, Fischer, 1899, p. 221. 
