MUSEUM OF COMPARATIVE ZOOLOGY. 119 



marked example of a very common, although not universal, condition 

 of the lateral margin of the colony. The epithelium of the margin is 

 composed of columnar cells, which are higher (51 //.) than those of the 

 roof (48 fx), and also of a less average diameter (8.4 fx) than the latter 

 (18.2 n). Moreover, the cells are very little metamorphosed. In pass- 

 ing towards the roof (tct.), the cells are seen to become more and more 

 metamorphosed, the secreted bodies (cp. sec.) becoming relatively larger. 

 Figure 55 represents the margin in a more metamorphosed state than 

 Figure 60. Although this condition of things is not incompatible 

 with the idea of a passive margin, it strongly suggests that this region 

 is one of proliferation, by which cells are added to the roof, and thus 

 the distance from the youngest polypide to the margin is virtually 

 increased. This conclusion receives a very important confirmation 

 from the study of the origin of the radial partitions, the treatment of 

 which must be deferred for the moment. Although new cells are being 

 added to the roof at the margin, yet the distance from the youngest 

 polypide to the margin is not greater in old than in young colonies. 

 How, then, is the approximate constancy of this distance maintained 1 

 Evidently it can only be by the process (which I have already shown 

 must take place) of migration of some of the young buds at. the base 

 of the ectoderm, particularly in the case of median buds. The ten- 

 dency of the migration of young buds towards the margin is to diminish 

 the distance between the front of the budding region and the margin 

 of the colony. The tendency of cell proliferation at the margin is to 

 increase that distance. The actual distance is the resultant of these 

 two opposing factors, and may be less or greater in different parts of 

 the same colony, according as the one or the other is the more active. 

 If we assume, further, that the cells added to the roof and sole from 

 the margin plus those derived from the necks of the polypides are 

 equal in amount to those lost by the degeneration of individuals in the 

 middle of the colony, we have a sufficient explanation of the fact, 

 observed long ago, that the adult colony of Cristatella maintains a 

 nearly constant width. 



7. Origin of the Radial Partitions. — I know of nothing on this sub- 

 ject by any previous author. The radial partitions consist of a muscu- 

 laris covered on both faces by a very thin epithelial layer (Plate X. 

 Fig. 95, 1). The muscle fibres of the muscularis arise from the already 

 formed longitudinal muscles of the wall of the colony at the region of 

 transition from the sole to the roof (Plate VI. Fig. 55, mu.). As the 

 muscle fibres move into the coenoccel, they carry before them the ccelo- 



