MUSEUM OF COMPAKATIVE ZOOLOGY. 145 



carry the body-wall at the region of the atrial opening to a considerable 

 height above the level of that portion of the roof lying between polyp- 

 ides. (Compare Fig. 73, Plate VIII. ; Figs. 98 and 99, Plate XI.) This 

 method of origin of the body-wall is of much less importance in Crista- 

 tella than in Alcyonella, since the extent of the proper body-wall about 

 the atrial opening is much less in the former than in the latter case. 



The development of the gelatinous bodies deserves further attention. 

 Kraepelin ('87, p. 24) concluded, from a study of the condition in a 

 statoblast embryo, that they are formed by a metamorphosis of the cell 

 piotoplasra, beginning at the outer end of the cylindrical cell, and 

 finally involving, in some cases, the entire cell, together with its nucleus. 

 Some appearances which I have noticed in the ectoderm of Cristatella 

 lead me to conclude that the origin is not always so simple as Kraepelin 

 describes. Figure 79, Plate IX., shows at cp. ser. a niimber of small gelati- 

 nous masses occurring at various regions in the protoplasm. Such an 

 appearance is quite common, and must be interpreted, it seems to me, 

 as the formation of the gelatinous balls by an intra-cellular metamor- 

 phosis of the cytoplasm. The balls, flowing together, produce the larger 

 masses. The metamorphosed matter from several cells may also fuse 

 into one mass (Plate VI. Fig. 55, cp. sec). The final result of this pro- 

 cess of cell metamorphosis in the ectoderm is a frame-work of old cell 

 walls, having a thin layer of protoplasm and nuclei at its base, and in- 

 closing the great gelatinous balls. Such a condition exists near the 

 centre of the colony between adult polypides, and is shown in Figure 100, 

 Plate XI. 



Summary. 



1. Most individuals give rise to two buds, of which one forms a new 

 branch, the other continues the ancestral branch. 



2. The median buds migrate away from the parent polypide to a con- 

 siderable distance before giving rise to new buds. 



3. The descendants of equal age from common ancestors are arranged 

 similarly in the same region of the colony. 



4. New branches are formed upon either side of ancestral branches. 



5. The greater the difference in age between the youngest and the 

 next older bud, the greater the distance between the points at which 

 they begin to develop. 



6. In typical "double buds," both polypides arise from a common 

 mass of cells at the same time. From the neck of old polypides a stolon- 



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