MUSEUM OF COMPARATIVE ZOOLOGY. a) 
the distal end of the stalk, where it is a layer of cuboidal cells, to the 
base, where its cells are extremely flattened, so that the layer is hardly 
distinguishable. ven near the base of the stalk, however, the ectoderm 
is a relatively thick layer on each of the surfaces of the transverse septa. 
The axial portion of the stalk presents very interesting histological 
conditions. In a young individual whose stalk comprises a single seg- 
ment only (Plate II. Fig. 15) the axial portion contains a closely packed 
mass of cells, which stain deeply, contain few vacuoles, and are slightly 
elongated in the direction of the axis of the stalk. Ata later stage one 
finds the plasma of some of these cells undergoing a metamorphosis. 
Such cells appear filled by a vacuole in which lie numerous highly re- 
fractive granules. A little later all of the axial cells have become greatly 
elongated in correspondence with the elongation of the stalk. Many of 
them appear almost completely vacuolated. A cross section of the stalk 
at such a stage of development is shown in Plate II. Figure 9, where the 
centre is filled with a network of cell boundaries enclosing clear spaces. 
This condition is very striking in longitudinal sections through the re- 
gion of the septum, where the long vacuolated cells may be seen extend- 
ing from one segment into the other. 
According to Ehlers (’90, pp. 18-22), the stolon of Ascopodaria shows 
a condition of the axial portion similar to that here described ; but in 
the stalks which support the calyces there is a considerable amount of 
intercellular substance, and I find the same thing in Pedicellina. In 
Urnatella, on the contrary, such intercellular substance appears only in 
the older stalks. 
The musculature of the adult stalk is situated immediately within the 
ectoderm, and is found in all segments running from one septum to the 
other. On cross section of the stalk it appears as a circle of radially 
elongated, highly refractive bodies, placed close to the ectoderm (Plate 
II. Fig. 9, mu). Each elongated body has, at a certain focus, a darker 
periphery and a lighter central line. With the highest powers the darker 
periphery can be resolved into a single row of bodies, which appear as 
bright spots at the highest, as dark spots at the lowest focus (Plate II. 
Fig. 16, mu). These I interpret as fibrille. 
The fibrilla develop in the protoplasm of the axial cells. In their 
earliest stages one sees them forming a single row on the outer edges of 
the more peripheral cells (Plate II. Fig. 17, mu). Later they are formed 
on the two lateral edges also (Fig. 11). Additional lateral (or, in rela- 
tion to the stalk, radial) rows of fibrille are next formed in each cell, 
the new rows appearing nearer the centre of the cell than those already 
