98 
lopmental processes through which the ends are reached seem widely 
different. One recalls, as MAURER long ago showed, that the nume- 
rous alveoli of the bony fish’s thyroid are derived by budding from 
one original small alveolus which is formed as a hollow outgrowth 
from the pharynx floor soon pinching off to give a closed sac. The 
first striking point of difference is the great extent of the pharyngeal 
outpushing in Bdellostoma producing instead of the hollow sphere a long 
chain of cell groups. The breaking up of this chain into the cell 
groups from which the alveoli arise might be considered coimparable 
to the budding which takes place in the short spherical anlage of the 
bony fish for here also part of the cells are thus splitting away or 
breaking down. The one process might be considered a diffuse while 
the other is a definite method of obtaining the same end result. 
There is nothing in the development of the thyroid in Bdello- 
stoma to suggest a paired origin for this organ, as is found to be 
the case in some animals. 
Amphioxus and Ammocoetes have the evaginated glandular thyroid 
bodies forked and retaining their original connection with the pharynx. 
Chietly on this account WILHELM MÜLLER in 1871 homologised the 
thyroid gland with the hypobranchial groove of the tunicates. Lately 
Reese!) has described the thyroid gland in Ammocoetes as arising 
from a groove which becomes shunted off by a roof folding over its 
anterior and posterior ends. The tube thus formed opens into the 
floor of the pharynx where the peribranchial ciliated grooves run dor- 
sally to form the dorsal ciliated tract which passes caudad to the 
oesophagus. This originally simple thyroid tube becomes continuously 
more complex by folding its walls to form -glandular tracts which 
communicate with the general cavity and finally open out through the 
pharyngeal oriface into the throat. After metamorphosis the thyroid 
decreases in size, and in old sea lampreys REESE states that it is 
almost if not entirely lacking. The throat connection is always lost 
in Petromyzon and the gland becomes a number of closed follicles 
imbedded in a mass of surrounding cells. The only point of striking 
similarity to Bdellostoma found in such a description is the groove 
like anlage in Ammocoetes. It may further be concluded that the 
Ammocoetes condition seems to approach nearer the primitive one 
than does that of the Bdellostoma embryo. In other words the 
thyroid development in Bdellostoma fails to furnish any conclusive 
1) Structure and Development of the Thyroid Gland in Petromyzon, 
Proc. Acad. Nat. Scs. Phila. 1902, p. 85. 
