MUSEUM OF COMPARATIVE ZOOLOGY. 27 



stolon-like body, which will become a basis of support for the new stock, 

 the " basal plate." One can thus account for the thickened cells of 

 the under side of the stolon, which appear before detachment (Plate VI. 

 Figs. 51, 57). The question remains, Do the median branches play a 

 similar role to the stolons'! I do not think they do, for the reasons, 

 (1) that, having no basal plate, they are not physiologically fit for form- 

 ing new stocks; (2) that I have found no new young stocks having 

 one parent stem with one or two generations of budded individuals, — 

 the condition of the median branches ; (3) that, on the contrary, one 

 often tinds such median branches persisting on even the lower segments 

 of the stock. (Plate V. Figs. 35, 39, 40. Compare Leidy, '84, pp. 8, 9, 

 Plate 1. Fig. 4.) Since the median branches frequently persist as a part 

 of the parent stock, — they are not produced in the lirst place on every 

 segment, — I conceive their function to be the increase of the number 

 of proliferating points in the stock itself. 



Starting with the young stock, one can find all stages of growth up to 

 the most complicated conditions (Plate V. Figs. 33, 36, 43, 44). Dur- 

 ing the growth of the stock the basal plate gradually undergoes changes. 

 The parenchyme becomes filled with yolk globules (Plate V. Fig. 49), 

 and the cuticula becomes thick and dark. 



Concerning the morphological significance of the basal plate a few 

 words must be said. I regard this as a stolon morphologically equiva- 

 lent to the stolon of the Pedicellinidse. In the latter group, as is well 

 known, the individuals are budded from the upper side of a repent cy- 

 lindrical stolon, which constantly produces new buds at the growing end, 

 and which becomes separated into segments by the formation of trans- 

 verse dissepiments. There is no such stolon in the adult Urnatella, 

 which is sharply separated from the Pedicellinidse by this single charac- 

 ter. The presence of a stolon in the young stock indicates a derivation 

 from an ancestral condition possessing a stolon in the adult. 



If, however, the " stolon " of the young Urnatella stock is homolo- 

 gous with that of the Pedicellinidse, we ought to find it, sometimes at 

 least, giving rise to more individuals than two, and perhaps becoming 

 segmented. Both of these conditions are occasionally fulfilled. Leidy 

 observed that three, four, or even five stems may arise from a com- 

 mon " basal plate." I have observed only three with certainty. Two 

 cases of this are shown in Plate V. Figs. 48 and 49. In the first of 

 the two cases distinct perforated dissepiments were observed dividing 

 the stolon (basal plate) into three segments, out of each of which a 

 single stalk arose. 



