294 University of California Publications. [Zoology 



Metamorphosis is accomplished (1) by a shift of the oral 

 boundary of the perisarc through a transformation of the ecto- 

 derm of the column, (2) by the development of a fenestrated 

 membrane through local growth and fusion of epithelial cells, 

 followed by a lateral tension; the ultimate fenestration being 

 assisted probably by the currents in the subjoining canals, (3) 

 by the appearance of the peripheral canals, due to the local 

 growth of the endodermic epithelium on certain longitudinal 

 lines and the expression of cells into the enteric cavity. The 

 canals maintain clear passages by internal pressure. Their 

 number and size bear a relation to the diameter of the stem. 

 (4) By growth of the stem, which is largely due to the immense 

 increase in volume of the axial tissue by the absorption of water. 



The axial tissue is renewed by the displacement of epithelial 

 cells from the peripheral canals. It acts as a skeletal axis, in 

 the absence of a supporting perisarc. 



The division of the hydranth cavity into pre- and post-ten- 

 tacular portions is connected with the growth of the proximal 

 tentacles. 



The gonosome is produced by an evagination of both layers. 



The most conspicuous characteristic of the development of C 

 palma appears, from the facts just enumerated, to be its plas- 

 ticity. Long after the close of the segmentation period, when 

 the embryo possesses both an epithelial ectoderm and a mass of 

 endoderm cells fast taking the form of a lining for the develop- 

 ing coelenteron, it still remains for external conditions to fix the 

 oral and aboral poles of the body. This result having been 

 accomplished, the delimitation of hydranth and stem is marked 

 out, before the appearance of a tentacle or frustule, by the sub- 

 tlest of transformations. A circular depression in the ectoderm, 

 produced by a decrease in the size of certain cells, and a circular 

 ridge in the endoderm produced by a swelling of certain other 

 cells, without any trace of cell division, are the visible signs of 

 the differentiation. Subsequently, all the characteristic organs 

 — tentacles, frustules, peripheral canals, axial endoderm — are 

 moulded in large measure out of more or less differentiated epi- 

 thelial tissues without recourse to residual cells. 



