Richard Mills Pearce 449 
and excellent preservation of this specimen adapted it for a most careful 
and accurate study of the relations and differentiation of the cells. The 
glands are still represented by branching processes of cells, lying in an 
abundant stroma, but here and there a few processes which have become 
tubular are seen. The cells are closely crowded together, especially at 
the periphery or growing portion of the process. ‘They have rather 
deeply staining, round or oval vesicular nuclei, and a very small amount 
of shghtly granular protoplasm. The protoplasm is readily demon- 
strable in those cells forming 
tubular processes; the nuclei are 
seen at the periphery, while the 
clear protoplasm at the ‘attach- 
ment of the cells to the base- 
ment membranes forms a ring 
about the lumen. Here and 
there, and generally at the per- 
iphery, in either the solid or tu- 
bular processes, is occasionally 
seen a much larger cell with 
deeply staining chromatin and 
either with clear or eosinophilic 
protoplasm. These cells corres- 
Hie. 1. EBucrCHS of an embryo 54 ms in length 
S ry 7 4). Early stage of the differentiation of an island o 
pond to those described by La Langerhans. A round mass composed of cells 
ouess § r rich in protoplasm is seen continuous with the 
guesse and Renaut, but I have eT Rpe ran ho Sd. 
not been able to convince myself 
that they represent the primary differentiation escare to the formation 
of the islands. They always occur singly and the arrangement of the 
chromatin is suggestive of karyokinesis. Entirely distinct from them 
are the small groups of cells lying at the side of a glandular process, but 
in direct continuity with it (Fig. 1). Each group is composed of ten 
to fifteen cells which have a round, lightly staining nucleus, and a com- 
paratively large amount of finely granular protoplasm staining deeply 
with eosin. The cells are closely applied one to the other and form round 
or oval masses which can readily be distinguished from the acinar pro- 
cess from which they arise. Sometimes they lie in a semilunar projec- 
tion of the gland without distinct connections; but in following the 
group through a series of sections, the continuity with the gland can 
generally be demonstrated. 'The masses are generally distinctly sepa- 
rated from the surrounding connective tissue by a narrow space, which 
gives the effect of a capsule. This differentiation of cells represents, [ 
believe, the earliest stage of the development of the islands, and is identi- 
