ON THE DEVELOPMENT OF TUBULIFOKA. 147 



bule at first sight seem to indicate that the walls of the space 

 represent the tentacle-sheath. There is, however, one fact 

 which suggests a different explanation. The distal end of the 

 wall of the embryophore in stages D and E is invariably 

 reflected inwards, as shown in figs. 22 and 25, the reflected 

 part usually terminating in cells which are loosely connected 

 and lie in the cavity of the embryophore. This relation is 

 very similar to that of what I have called above the somatic 

 mesoderm of the young polypide-buds (fig. 23, s. m.). If the 

 wall of the embryophore-cavity can really be compared with 

 this layer, the general history of the ovicell would appear to 

 be somewhat as follows. The ovicell is at first an ordinary 

 zooecium, whose first polypide develops a functional alimentary 

 canal and an ovary. An egg begins to develop, probably while 

 the polypide is still functional, but still comparatively young. 

 The polypide then degenerates and forms a brown body. 

 Certain cells arrange themselves as a definite investment round 

 the brown body; and these cells, whose origin is obscure, 

 probably represent a polypide-bud. A vestibule makes its 

 appearance distally to the brown body. The body-cavity of 

 the ovicell-bud appears as a space just inside an outer epithe- 

 lial layer, which represents its somatic mesoderm. This layer 

 lies freely in the old body-cavity, and its outer surface may be 

 more or less covered with cells belonging to that cavity. The 

 greater part of the rudimentary polypide-bud of the ovicell 

 becomes a mass of nutritive tissue, in the meshes of which the 

 secondary embryos are afterwards contained. The brown body 

 comes to lie freely in the new body-cavity by the breaking up 

 of the nutritive tissue. By the enlargement of the body-cavity, 

 its somatic mesoderm is brought close to the outer calcareous 

 wall of the ovicell, and the new body-cavity thus replaces the 

 old one. The cavity becomes lobed in correspondence with 

 the lobing of the entire ovicell. The lumen is visible distally, 

 even in advanced stages, while it becomes filled up proximally 

 by the great development of the nutritive tissue and of the 

 secondary embryos. One of the lobes of the ovicell, the 

 " axial lobe," is connected with the exterior by means of the 



