EK. T. Bell 61 
separate large areas of clear cytoplasm, causing the appearance of large 
epithelial cells. Colloid is being formed between the lamelle and around 
several nuclei. 
Fic. 4. From a 8.5-em. pig embryo. Stained with iron-hematoxylin (not 
decolorized). The medulla has appeared. Lymphocytes are present between 
the epithelial cords. 
Fig. 5. From a 7-cm. pig embryo. Stained with iron-hematoxylin and 
Congo red. A few lymphocytes have been formed. In the cellular reticulum 
are large pale nuclei, lymphoblasts, and large dark intermediate nuclei. The 
nuclei in mitosis are very compact. 
Fic. 6. From the medulla of a 24-cm. pig embryo. Stained with Jackson’s 
modification of Mallory’s method (ref. in text). Many fibrille are seen in 
the syncytium. 
Paar ul: 
Fic. 7. Ordinary type of simple concentric corpuscle. From a 16.5-cm. 
pig embryo. Stained with haematoxylin and Congo red. The corpuscle is 
well advanced in development. Concentric lamelle of colloid have been 
formed. The cytoplasm between the lamelle is in an early stage of colloid 
transformation. Colloid fibers cut transversely appear as dots. The nuclei 
are becoming flattened by the pressure of expansion. The central mass stains 
irregularly and all traces of the nuclei in that region are gone. 
Fic. 8. Two cystic concentric corpuscles. From a 16-cm. pig embryo. 
Stained with iron-hematoxylin and Congo red. On the left, a nucleated 
mass of protoplasm has been separated off by the formation of a vacuole 
annular in section. This might be mistaken for a blood-vessel containing a 
nucleated red cell. In this central protoplasmic mass the nucleus is shrunken 
and the cytoplasm vacuolated. In the small corpuscle on the right, two large 
vacuoles have formed. 
Fic. 9. Cystic concentric corpuscle. From a 14-cm. pig embryo. Stained 
with hematoxylin and Congo red. The central protoplasmic mass is pale 
and shrunken. The small circular body in it probably is the remains of 
the nucleus. Colloid lamelle are forming. Colloid fibers cut transversely 
appear as dots. 
Fic. 10. Cystic concentric corpuscle. From a 10.5-cm. pig embryo. 
Stained with hematoxylin and Congo red. The center contains no colloid 
and seems to be softening. The nucleus is shrunken. 
Fic. 11. Ordinary concentric corpuscle in a very early stage. From a 
10.5-em. pig embryo. Stained with iron-hematoxylin and Congo red. The 
nucleus is enlarged and colloid is forming around it. A few colloid fibers 
may be seen in the cytoplasm for some distance from the central nucleus. 
Fic. 12. Ordinary concentric corpuscle. Several nuclei are involved. 
From a 10.5-cem. pig embryo. Stained with hematoxylin and Congo red. The 
deeply-staining colloid has completely obliterated the central nucleus (in the 
region 0 c), and nearly obliterated another (n’). Some of the colloid now 
stains pale (0c). 
Fic. 13. Ordinary concentric corpuscle.. From a 10.5-cem. pig. Stained 
with hematoxylin and Congo red. The central nucleus is being obliterated 
