144 BULLETIN OF THE 



such a process is seen at Figure 98. By the time the atrial opening is 

 established these cells have become plainly muscular (Plate IX. Fig. 79). 

 Farther up in the angle of attachment of the kamptoderm to the roof 

 of the colony, the ccelomic epithelium and the outer layer of the bud are 

 both seen to be somewhat disturbed (Fig. 97, arm. su.). At different 

 points, a single one of these cells reaches across, and later becomes 

 differentiated into a genuine muscle cell (Fig. 99, mu. su.). Of these 

 there may be three rows. 



9. Disintegration of the Neck of the Polypide. — The neck of the polyp- 

 ide, having fulfilled its function as the most important part of the stolon, 

 must now give way to allow of the extrusion of the nearly developed pol- 

 ypide. The first indication of this process is the formation within the 

 cells of the neck of a secreted substance (cp. sec.'), apparently like the se- 

 creted bodies of the ectoderm. This metamorphosis first involves the outer 

 and middle cells of the neck only (Plate XI. Fig. 97, cev.pyd.). Later 

 (Plate IX. Fig. 77, of. atr.) a depression occurs in the ectoderm. This is 

 due, I believe, to a cessation of cell proliferation at the centre, although 

 it remains active at the edges of the neck. The depression gradually 

 deepens until the atrium is closed by a thin layer of cells only (Fig. 98). 

 The cells of the side of the neck do not disintegrate, but go to form the 

 " Randwulst" of Kraepelin ('87, p. 40). The cells of this region remain 

 unmetamorphosed. Only a thin layer of cells now stands between the 

 polypide and the outside worlfl. This ruptures, as is shown in Figure 

 99, and by the relaxation of the muscularis, which is thickened about 

 the atrial opening into a sphincter (Fig. 98, spht.) t the polypide is ready 

 to expand itself. 



10. Development of the Body-wall. — As already stated (page 117), 

 Braem believes that the whole body-wall in Alcyonella is derived from 

 the neck of the young polypide, after it has begun to give rise to 

 daughter polypides ; and I have given my reasons for believing that in 

 Cristatella a portion of it at least is derived from the margin. 



In addition to this, cells are undoubtedly added to the body-wall, as 

 Braem states, after the time of origin of the buds. Particularly after the 

 formation of the median bud, the neck appears to continue to furnish cells 

 to the ectoderm. Figure 73,* Plate VIII., shows such a mass of cells. 

 Later stages show that these cells secrete a gelatinous substance within 

 their protoplasm (cp. sec.', Plate XL Figs. 97, 98) ; they gradually in- 

 crease in width and height from the neck outward (Figs. 97-99), and at 

 the same time become more and more completely metamorphosed. The 

 result of the addition of these cells from the neck of the polypide is to 



