1910] Torrey: Regeneration of Eydranth and Holdfast. 211 



thickens slightly at these points, its cells increasing in number 

 .•md length. The mesogloea thins somewhat, dissolved probably 

 by the secretion of the endoderm cells. 6 



Differences between tlie normal and regenerative development 

 of the tentacles now begin to appear, thai are referable to the 

 relative complexity of the column in the adult. Instead of the 

 extrusion of a single row of cells from the simple endoderm, to 

 form the axis of the larval tentacle, a mass of cells is pushed out. 

 so that the tentacle, much broader from the beginning, reaches 

 the stage of histological differentiation characteristic of the adult 

 with exceptional directness. Viewed as changes involving mere 

 masses of tissue, the processes differ only iii magnitude. Viewed 

 as changes in the structure of the tissues, and with respect to 

 the behavior of the cells composing them, certain qualitative 

 differences are manifest at once. These are smaller for the distal 

 tentacles, whose development may be described in a few words. 

 A local proliferation of epithelial endoderm produces a small 

 group of cells that is pushed as it forms into a position below the 

 endoderm. carrying the mesogloea and ectoderm out with it. the 

 latter thinning slightly at the apex of the advancing endodermic 

 core (fig. 7). This core is shortly separated by a sheet of 

 mesogloea from the epithelium from which it has arisen, and 

 thereafter is the sole source of the cells in the axis of the tentacle. 

 A certain complication is introduced into this simple process 

 in the development of the proximal tentacles, correlated with the 

 simultaneous development of the annular cushion of vacuolated 

 endoderm that strengthens the hydranth wall in the region of 

 their insertion. 7 There is an enlargement and an increased 

 vacuolation of the cells lining the canal (or hydranth cavity, as 

 the case may be) at the point where the ectoderm has already 

 thickened preparatory to the budding of the tentacle; and at the 

 same time a layer of small cells makes its appearance below 

 them, immediately against the mesogloea (figs. 8, 9). This layer 

 is destined to produce the cells of tile core. Its cells multiply 

 rapidly and push out the ectoderm, assuming the form of 

 Stratified epithelial cells, compressed in the direction of the axis 



8 Cf . development of frustules, loc. cit. 

 "See fig. \17 in the paper already cited. 



