'_'1o University of California Publications in Zoology. [Vol.6 



tentacles of the embryo, the size of the fundament varying in 

 general with the diameter of the stem. 



hi tlie embryo, at the time tentacles begin to appear, there is 

 a single cavity, lined by endoderm cells which are larger and 

 longer than the cells of the ectoderm, and closely resemble in 

 size and variolation the axial cells of the tentacles. The axis 



Pig. 5. — Distal tentacles reg rating in quartettes. 



Pig. ii. — Regenerating segment of column mucb contracted. Wound not 

 \.i completely closed. Proximal tentacles indicated. 



of both proximal and distal tentacles arises as a single row of 

 cells which are pushed one after another out of the lining of the 

 hydranth cavity. This axial endoderm in course of development. 

 Incomes several cells thick at the bases of the tentacles, though 

 ElO ca\ities are formed. 



In the regenerating adult column, the single cavity of the 

 embryo is replaced by the peripheral canals, which are bounded 

 on three sides by the vacuolated cells of the core of the column, 

 and on the fourth ( outer I by an epithelium (fig. 2). The cells 

 of this epithelium measure about the same in height as those of 

 the epithelial endoderm of the larva. They are more granular. 

 however, and lack conspicuous vacuoles. The ectoderm is higher 

 than the endoderm. especially in the naked portion of the column, 

 and the mesogloea, almost invisible in sections of the embryo, has 

 become a prominent layer. 



The first indication of the regenerating tentacles in a surface 

 view, is an increased opacity, depending on an increased granu- 

 lation of the endoderm. at the points where the tentacles are to 

 appear I fie-. 6). At the same time, as sections show, the ectoderm 



