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same cut. The chromatin here forms a loose reticulum, and many nuclei 
contain vacuoles of various sizes. The cytoplasm is not clearly shown 
by the haematoxylin stain, and at this stage the cell bounderies could 
not be distinctly traced. The number of cells in the groups vary 
from a single cell up to eight or ten, but at this stage no group was 
seen to form a definite hollow sphere which is characteristic of the 
adult condition. 
Embryos slightly older than the above show no particular change 
in the general structure of the thyroid. The gland is still a long 
mass or chain of cells around which groups of cells are now scattered 
as if they had broken away or budded off from the general mass. 
Fig. 4 A group of nuclei from a thyroid that has formed a mass of cell groups, 
at this time the cell boundaries were not clearly distinguished. About 365 diameters. 
Fig. 5. A. Large cell group which will later break down into smaller groups. 
Ks. Chromatic nuclei. Nm. Heavy nuclear membrane. B. Two smaller cell groups 
from the same stage. 
Fig. 6. The cell groups seen in a verticle section as the original mass of the 
thyroid anlage is breaking down. v. Vacuoles within the nuclei. 
Fig. 7. A. Surface view and B. a section of a young alveolus with no surround- 
ing membrane. Ac. Alveolar cavity. 
Fig. 8. Alveolus about which the mesenchyme cells are beginning to form a 
“membrana propria”. Ac. Alveolar cavity. 
