97 
Fig. 5 A and B show the histological structure of the cells at this. 
stage, the nuclei are seen to be larger than in the former period and 
surrounded by a distinct membrane. Large chromatin nucleoli are 
present in most nuclei often more than one, and the chromatin forms 
a distinct reticulum. The cytoplasm is clear and lightly staining and 
the cell bounderies are now readily distinguishable. A stage some- 
what more mature is seen in Fig. 6 where the long cell mass is al- 
most completely broken down and has a much wider vertical distri- 
bution. Some of the groups in this gland have a clear refractive 
substance between the cells, leading us to conclude that the alveoli 
are forming, their cavities now being filled with colloidal substance. 
This embryo was rather poorly fixed so that the cells and nuclei are 
shrunken and their histology is not to be relied upon. 
Finally in an embryo that has hatched the thyroid is seen to be 
in almost the adult condition. It consists of diffusely scattered alveoli 
below the pharynx and above the median branchial artery. The al- 
veoli are of various shapes, generally spherical but often ovate, U 
shaped, or irregular in form. The central cavity is filled with a re- 
fractive substance. The alveolar walls are of a single layer of poly- 
gonal cells with the characteristic large nuclei. Fig. 7.4 and B show 
a surface view and a section of a young alveolus, the cell bounderies 
are clearly seen and no covering of fibrous cells, or “membrana pro- 
pria” as SCHAFFER terms it, has yet formed about the surface. In 
Fig. 8, a section of an older alveolus, one notes the mesenchyme cells 
apposing themselves to the surface, their nuclei flattening out and 
lying parallel to the curvature of the sphere. The older alveoli are 
also seen to be larger than the younger ones, those figured measuring 
respectively .03 mm. and .022 mm. 
Thus it is seen how the final adult condition is reached from the 
trough-like anlage of the embryo. The manner in which the groups 
of cells finally give the alveoli is strikingly analogous to the way in 
which the morula of a holoblastic egg passes into the hollow blastula 
stage. I was entirely unable to trace the division of the cells while 
producing the many celled alveoli from the original solid cell groups. 
Only one cell undergoing mitosis was seen in the many glands studied 
while in other tissues of the embryo numerous dividing cells were 
often found. From the manner in which the thyroid nuclei were at 
times grouped one would be inclined to expect amitosis though no 
direct evidence of this was obtained. 
It will now be clearly seen that although the adult condition of 
the thyroid in Myxinoids and Teleosts are readily comparable, the deve- 
Anat, Anz, XXIX. Aufsätze. 7 
