INTERNAL FACTORS OF REGENERATION IN ANIMALS 59 



If we examine the factors that determine the production of the 

 partial structures, we find, in the first place, that the size of the piece 

 is of the greatest importance. The reduced forms appear most often 

 in pieces that are shorter than the average length of the hydranth- 

 forming area. A second factor is connected with the region of the 

 stem from which the piece is taken. Larger pieces from the distal 

 end produce partial structures, especially hydranths with very short 

 stalks (Fig. 28, C), or with none at all (Fig. 28, D). There are cer- 

 tain facts connected with this distal region, which lies just behind the 

 hydranth, that should be mentioned in this connection. It was first 

 discovered by Dalyell that a hydranth-head lives for only a limited time, 

 and that when it dies a new head is regenerated from the region behind 

 the old one. The stalk of the new hydranth continues to elongate for 

 some time after the new hydranth has been formed. Whether this 

 continuous growth in the distal end, or the normal formation of a new 

 hydranth by it from time to time, can in any way be connected with 

 the development of partial structures from this region cannot at 

 present be stated. The distal part of the stem contains more of the 

 red-pigment, that gives color to the stem and to the hydranth, than 

 does any other part. Loeb first advanced the view that the red- 

 pigment in the stem acts as a formative substance in Sachs' sense, 

 and determines the production of a new hydranth by accumulating 

 near the cut-end of the piece. Driesch also assumes the red-pigment 

 to be a factor in the result, but supposes that it acts quantitatively, 

 rather than in determining the quality of the result. If this red-pig- 

 ment acted in the way supposed either by Loeb or by Driesch, it might 

 act as one of the factors in the production of these partial structures. 

 This red-pigment is contained in the form of reddish granules in the 

 cells of the endoderm. The granules are of various sizes, the largest 

 being easily seen even with low powers of the microscope. When a 

 piece of the stem is cut off, the ends close by the drawing in of the 

 cut-edges over the open-end. A circulation of the fluid contained in 

 the piece then begins. In the fluid, globules appear very soon that 

 contain red-pigment granules like those in the endoderm. The glob- 

 ules appear to be endodermal cells, or parts of cells, that are set free 

 in the central cavity. The circulation continues for about twenty- 

 four hours. At about this time one end of the stem becomes reddish, 

 owing to the presence in it of a larger number of red-pigment granules 

 than before. The ridges that are the rudiments of the tentacles appear 

 (Fig. 30, A), and a new hydranth very rapidly develops. At the time 

 when the hydranth begins to appear the globules in the circulating 

 fluid disappear. They disappear at the time when the red-pigment 

 of the forming hydranth is rapidly increasing in quantity, and not 

 unnaturally one might suppose that the pigment of the circulating 



