95 
Fig. 64, page 69 one sees a small group of large circular cells below 
the pharynx, the significance of which Kurrrerr fails to mention, 
although in position and outline they suggest very strongly some con- 
nection with the thyroid, but the other figures of this series make it 
appear very improbable that a thyroid structure would be present 
in such an early stage. 
The chief point of interest shown by the trough-like thyroid 
anlage is the very extensive gut area from which it is derived, in no 
other vertebrate does the thyroid evagination from the pharynx run 
through so relatively long an area, through it is equally true that in 
no other vertebrate are the gills so numerous or their area so 
extensive. It is just on account of the last mentioned condition 
that the thyroid is extensively elongated and does not for any ob- 
vious reasons show, as many might be inclined to believe, that this 
is primitive, indicating a close phyletic relationship to the endostyle 
of Amphioxus and the Ascidians on the ground that both organs oc- 
cupy similar areas. I would be understood not to deny the probable 
phyletic origin of the thyroid in vertebrates from an organ like the 
endostyle in Amphioxus, but rather to mean that this gland in 
/Baellostoma merely on account of its extent does not furnish ad- 
ditional evidence of this relationship and such a standpoint will be 
made necessary by following the further development of the organ. 
The next stage to be considered is that of embryos about 28 mm 
in length. At this time all of the gills have the pouches formed and 
the cartilage arches are beginning to be laid down about the external 
gill tubes. The brain is taking on the more compact condition of 
the adult, the large ventricles of the young embryonic brain being 
greatly reduced in extent. The thyroid is now to be described as a 
more or less continuous chain of cell groups, as if the body of the 
trough-like evagination after pinching away from the pharynx floor 
had begun to break down or disintegrate into small groups of cells. 
In point of fact even while the organ still retained its connection 
with the gut floor the body of it was seen in many regions to be 
made up of small cell-groups. The nuclei of all the cells in the 
thyroid tissue are much larger and deeper staining than those of the 
mesenchyme cells among which they lie. The thyroid cells at this 
stage stain just as the endoderm cells of the gut wall do. The thyroid 
cells extend anteriorly as far as the first gill pocket and run out 
posteriorly in the region of the last gill. In Fig. 4 is shown one of 
the cell groups from this stage, the nuclei are not so large as those 
from older glands as is seen by comparing them with others in the 
