MUSEUM OF COMPARATIVE ZOOLOGY. 5 



the distal end of the stalk, where it is a layer of cuboidal colls, to the 

 base, where its cells are extremely flattened, so that the layer is hardly 

 distinguishable. Even 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 veiy 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. At a 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 othei*. 



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 1 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 

 11. Fig. 9, niu). 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 11. 

 Fig. 16, rmi). These I interpret as fibrillse. 



The fibrillse 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, mn). Later they are formed 

 on the two lateral edges also (Fig. 11). Additional lateral (or, in rela- 

 tion to the stalk, radial) rows of fi brills) are next formed in each cell, 

 the new rows appearing nearer the centre of the cell than those already 



