54 The Development of the Thymus 
puscle is shown. ‘There are three simple concentric corpuscles in it— 
one of them (the lowest in the figure) in a very early stage. Several 
lamelle are common to the older corpuscles, and one is common to all 
three. This arrangement of the lamella is a mechanical effect of the 
tension in the cytoplasm, due to the centrifugal’ pressure from the two 
centers. ‘The size of the separate centers in a compound corpuscle de- 
pends upon the stage they have reached when they come in contact. If 
a compound corpuscle be formed by the union of two simple corpuscles 
in an early stage, as in Plate III, Fig. 19, all indications of its com- 
pound nature are soon lost. A corpuscle originally compound may, then, 
in later stages, become indistinguishable from simple corpuscles. The 
simple corpuscles uniting to form a compound concentric corpuscle may 
be of any of the types previously described. 
B. IRREGULAR CORPUSCLES. 
This group includes those corpuscles which are not at first concentric. 
Concentric areas may appear later. According to the classification pre- 
viously given, I distinguish a compact type and a reticular type. 
(a) The compact type (Plate III, Fig. 16) first appears as a compact 
area of syneytium of irregular shape. It is recognizable by the colloid 
it contains. The nuclei are not noticeably increased in size and have no 
regular arrangement. Their chromatin still stains dark with nuclear 
stains. The colloid (cf) is not yet solid. The corpuscle has no distinct 
center. These corpuscles grow by direct colloid transformation of the 
adjacent syneytium. No distinct lamellae are formed. The colloid may 
remain in the fibrous condition shown in the figure (cf) or it may 
become solid, but it never reaches the deeply staining condition unless ¢ 
concentric area be established. 
A later stage of this type is shown in Plate III, Fig. 23. The cor- 
puscle is sharply marked off from the syncytium. Some of its colloid is 
solid. A concentric area (cs) is beginning to form. The nuclei are 
not markedly different from those of the adjacent syncytium. These 
corpuscles may become large and branched. Often one or more con- 
centric areas are developed after the corpuscle has attained considerable 
size. By the growth of these concentric areas, irregular corpuscles may 
become converted into concentric corpuscles. 
(b) The reticular type is produced by colloid formation in the or- 
dinary reticulum of the medulla. In the types previously described, the 
spaces of the reticulum are usually obliterated as the colloid formation 
advances; but in this form the spaces persist as a part of the corpuscle. 
Pure reticular corpuscles vary greatly in size, sometimes involving only 
