290 University of California Publications. [Zoology 



until the stem has reached a diameter of about .25 mm. Two 

 to four arise at once, which rapidly subdivide into six to nine. 

 Plere they are found at their minimum diameter. From this 

 time they not only increase in number with the diameter of thd 

 stem, but the size of each canal and the number of cells in its 

 epithelial boundary also similarlj^ increase. 



Space relations appear to determine to some extent the numer- 

 ical development of the peripheral canals, by fixing the lines 

 along which the vacuolation and intrusion of the future axial 

 cells occur. The essential factor in the formation of the axial 

 parenchyma itself is the growth by absorption of water of epi- 

 thelial cells. In the first stages of this process, as we have seen, 

 the swollen cells meet across the cavity while retaining their 

 connection with the mesogioea. The subsequent extension of 

 the axis is due to continued growth of the cells composing it and 

 the addition of more cells from the epithelium in such a way 

 that the first formed vacuolated cells are pressed away from the 

 mesogioea and out of the intercanalicular septa by the growth 

 of their successors. The cells occupying the space within the 

 circle of peripheral canals are of enormous size, with exceedingly 

 thin walls and very slight traces of protoplasmic content. 

 There is no evidence that they increase among themselves. 

 Their number is doubtless renewed and augmented from the 

 intercanalicular septa alone, whose cells are themselves derived 

 from the epithelium of the canals. 



The peripheral canals are kept open by intercanalicular pres- 

 sure. Otherwise, the axial tissue does not appear to be sub- 

 jected to any restraint except that imposed by the length and 

 diameter of the stem. A positive intracellular pressure, then, 

 is the chief factor in the growth of the axial endoderm, and 

 must be reckoned also among the main causes of the growth of 

 the stem. 



This view is born out by various facts. The acceleration of 

 the growth of the stem, first, with the appearance of conspicuous 

 vacuoles in the endoderm of the early larva and second, with 

 the formation of the axial endoderm, has been mentioned above. 

 It might be emphasized further, that in the hydranth, where 



