ORIGIN OF GASTRIC GLANDS OF ACANTHIAS 359 
During the extension of a glandular outgrowth down into 
the mesodermiec tissue it meets with a small amount of resistance 
for two reasons. In the first place, the constituent cells of an 
outgrowth are not anchored fast by any structure comparable 
to a basement membrane, and in the second place, the meso- 
dermic tissue is of such a loose character that it is simply pushed 
back and more or less condensed by the burrowing cells (figs. 4, 
556, amd 7). 
E. Rotation of cells 
Figure 6, from an embryo 137 mm. long, shows a glandular 
outgrowth at its maximum length. On comparison with figure 
5, it is apparent that the staining reactions of both the nuclei 
and cytoplasm are identical. The morphological features of 
the nuclei are also the same. The glandular outgrowth is no 
longer a tubular structure; the shape of its proximal end has 
been characteristically modified. This change of form may be 
ascribed to a rotation of cells in the lower two-thirds of the out- 
growth. Although cell boundaries are not distinguishable in 
this figure, it is apparent on comparison with figure 5, in which 
instance cell boundaries are also absent, that in following the 
course of the nuclei in figure 6, as compared with those shown 
in figure 5, one is simply tracing out the movements of the cells 
that contain them. A number of cells, as represented in figure 
6, have rotated through an angle of about 90°. The completion 
of the rotation process is seen in figure 7 (from a specimen 146 
mm. long). In this stage cell boundaries are evident; the long 
axes of the nuclei, instead of being placed parallel to the long 
axes of the future gland, as they were in the tubular outgrowth 
represented in figure 5, are now placed at right angles to the 
lumen of the gland. Secammon (15), in the course of a study 
devoted particularly to the histogenesis of the selachian liver, 
describes a similar rotation of cells in tubule anastomosis. He 
does not think that the nuclei shift their axes within the cells, 
for he frequently finds, in cases where faint cell boundaries can 
be made out, that the cells show the same changes in position 
as do the nuclei. 
THE JOURNAL OF MORPHOLOGY, VOL. 32, NO. 2 
