426 
F. A. rOT'I'S. 
that of any othei’s in my possession to an exact study of the 
histology. It will be best to first consider the conditions at 
the proliferating lip (pr. 1 .) Avhere fresh stolons are being 
formed (PI. 23 bis, fig. 13). The epiblast oP the cushion is at 
first of a normal type, narrow columnar cells with small 
nuclei containing scattered granules of faintly staining chro- 
matin. There then occurs a distinct furrow, which separates 
this region from the triangular proliferating lip. Here the 
cells alter their character, the nuclei becoming larger and 
rich in darkly staining chromatin, generally massed in two 
(or sometimes more) nucleolar masses. Several cells, markedly 
lai-ger than the rest, are to be met with in which there is a 
single large nucleolus. These changes are observed in the 
epithelium of the upper surface only ; the cells of the lower 
surface remain short and small, with faintly staining nuclei 
passing again at the trough of the furrow into columnar cells. 
The mesoblast of the interior consists of very numerous 
cells, embedded in a fibrous matrix. The cells are frequently 
spindle-shaped. The chroimitiu of the nucleus consists of a 
single deeply staining nucleolus and other smaller granules, 
and mitoses are relatively common, while they are rare in the 
epiblast. As the mesoblast enters the proliferating lip, and 
is traced dorsally, changes occur similar to those noted in the 
epiblast — the cells increase in size and the nucleolus tends to 
increase in bulk and staining qualities. But though under- 
going similar modification there is nearly always a distinct 
line between ectoderm and mesoderm, which is never crossed 
by migrating cells. 
Next to the proliferating lip is the youngest stolon (.si'.), pear- 
shaped, Avith a narrow stalk, and as yet Avithout an}^ begin- 
nings of segmentation. The histological characters are like 
those described in the preceding paragraphs, and it is sug- 
gested that young stolons of this character are occasionally 
cut off from the proliferating lip. But even at this early 
stage there is a considerable difference betAveen the tAVO sur- 
faces of the young stolon. The upper surface {vent.), Avhere the 
epiblast is greatly thickened, becomes the ventral surface of 
