8 BULLETIN OF THE 



will arise ; and it is necessary to do this in order to study the origin of 

 the gemmiparous cells, and the changes which they undergo preparatory 

 to an actual involution. 



The study of tips of branches shows that the necks of the polypides of 

 any branch all lie in one plane, and that this plane also includes the 

 youngest polypide ; also that the youngest polypides always arise distad of 

 the next older. Knowing these facts, our observations may be confined to 

 a short line running from the neck of the youngest apparent buds Jo the 

 tips of the branches studied. The time at which to search for incipient 

 buds and the place in the line where they will be found is illustrated by 

 Figure 7 (Plate I.). The youngest developed hvvi. is one the axes of wliose 

 tentacles are approximately parallel to the axis of the branch, and whose 

 brain cavity, gii., is not yet constricted off' from that of the oesophagus. 

 The place of origin is near the tip, immediately beyond the point at which 

 the ectoderm changes rapidly from a columnar to a pavement epithelium. 



Figure 3 is from a section across the branch in the region of an incip- 

 ient bud. I have already described the conditions of the cells of this 

 region. Those near ex. are larger than the surrounding ones, and show 

 signs of cell division both in the ectoderm and mesoderm. In both cases 

 shown in the figure, the direction of division is such as will tend to 

 increase the superficial area of the layer in which it occurs. The ecto- 

 derm seems to be the most important layer of the two in the process of 

 invagination which is about to take place. I think one is led to this 

 conclusion if one considers a folding of an epithelium to be due to an 

 increase in the area of the epithelium within a certain circumference 

 without a correspondingly great increase in the circumference itself. 

 Such a conception implies, first of all, mutual pressure of the cells of 

 the invaginating epithelium. The cells of the mesodermal layer do not 

 seem to be under mutual pressure ; in some cases they are barely in 

 contact. The cells of the ectoderm are evidently closely applied, and 

 probably, therefore, under mutual pressure. 



The one case of coll division which is occurring in the ectoderm is at 

 the inner end of the cell. In fact, the centre of the nuclear plate is much 

 nearer the deep end than are the centres of the adjacent nuclei. The 

 efTect of this division is to increase the area on the inner surface of the 

 ectoderm more than that on the outer, as appears from a study of the 

 sections shown in Figures 4 and 5. In Figure 4 certain cells lie already 

 below the niveau of the surrounding ones, very much as though they had 

 moved downw-ard on account of this being the direction of least resist- 

 ance. A later stage of this process is shown in Figure 5. Here the 



