ABSORPTION OF THE HYDRANTH IN HYDROID POLYPS. 30 1 



the time, due to the growth of the stolon, but the protoplasm of 

 the polyp cannot apparently be draivn through into the stem 

 until it has reached a certain stage in its absorption. 



The finer structure of normal Campanularia is as follows : 

 The ectoderm cells which are flat on the body become cubical 

 on the hypostome ; there are no nettle cells except an occasional 

 wandering one, until we come to the upper half of the tentacles. 

 Below the cup lie masses of nettle-forming cells, somewhat 

 irregular in their position, but never found in an quantity anterior 

 to the first annulation. The endoderm is well differentiated on 

 the hypostome into deeply-staining goblet cells and long spindle- 

 shaped cells ; in the walls of the body cavity there are large, 

 clear endoderm cells and smaller granular gland cells. The ten- 

 tacles contain a single row of endoderm cells. These are sepa- 

 rated from those of the body cavity by a lamella at the base of 

 the tentacle. Signs of change first arise in the endoderm of the 

 body and the digestive current becomes filled with degenerating 

 endoderm and gland cells, pinched-off portions of cytoplasm and 

 loose nuclei. This process continues for some time without the 

 appearance of any other change, except that as the endoderm 

 becomes less, the lamella slowly contracts, becoming corre- 

 spondingly thicker, and the ectoderm, having less surface to 

 cover, changes from a thin layer to a much thicker one. The 

 tentacles have also contracted to an abnormal extent, and at last 

 by the breaking of the lamella across their base the endoderm 

 cells round up and pass out into the body cavity. At this stage 

 the tentacles are crowded together, and, the ectoderm being 

 thrown into folds by the excessive contraction, frequently give, 

 in surface view, the effect of being fused, as stated by Loeb. But 

 by careful study the independence of the tentacles can be traced 

 in spite of the closeness with which they are pressed together. 



Soon after the endoderm has begun to pass out from the ten- 

 tacles the lamella breaks near the tip and masses of nettle and 

 ectoderm cells are poured into the cavity. The hypostome also 

 degenerates, the ectoderm cells passing out rapidly into the diges- 

 tive current and the lamella contracting after them. Soon the 

 lamella of the hypostome breaks and disappears and the mass of 

 ectoderm is also turned in. The polyp is now simply a shell of 



