F. B. Gumings — Development of Fenestella. 171 



Development of Fenestella. 



The Protoecium. 

 Many well-preserved Fenestella bases show a minute circular 

 pit on their basal surface. This can be seen only in colonies 

 that were attached to a substratum which disappeared in the 

 process of fossilization, leaving the basal surface of the colony 

 free from all extraneous matter. Where the colony is still 

 attached to the substratum, frequently the frond of another 

 bryozoan, the circular pit can always be demonstrated by 

 means of thin sections. This pit is the protoecium. As will 

 be seen from the longitudinal sections (figs. 20, 36, 37, 59), 

 the protoecium is separated from the substratum by a thin 

 basal membrane. In such sections, this pit appears as a semi- 

 circular object in the proximal portion of the colony. In trans- 

 verse sections, it appears as a dark ring surrounded by concen- 

 tric zones of punctate secondary sclerenchyma. That the 

 protoecium has its own proper wall, similar to that of ordinary 

 zocecia, is shown by numerous sections (figs. 36-38, and 59). 

 The diameter of the protoecium is from 0*4— 0*6 mm , or about 

 three or four times that of the ordinary zocecia. In form and 

 position it corresponds precisely to the basal disc of Cyclosto- 

 mata, and there can be little doubt that it has the same 

 morphological and developmental significance. 



The Ancestrula. 

 The protoecium is surmounted by a tubular structure arising 

 from the center of its distal surface. This is the ancestrula. 

 In some of the earlier sections prepared by the writer, one of 

 the primary buds was mistaken for the ancestrula, and its size 

 and shape were therefore thought to be different from what 

 was shown in later sections. It is considerably smaller than 

 the primary buds, being both shorter and of less diameter. It 

 seems altogether likely that the primary polypide never per- 

 manently ascended into the ancestrula as in the Cyclostomata. 

 On the other hand, the ancestrula of Fenestella is far from 

 being the homologue of the vestibule of ephebastic zocecia. 

 It is not built up of secondary deposits, but is composed of the 

 same thin non- punctate substance as the proper wall of the 

 protoecium and other zocecia. The homology of the ancestrula 

 of Fenestella is with the tubular primary zocecium of the 

 Cyclostomata. Figures 59 and 60 indicate the shape and 

 appearance of the ancestrula t as seen in the majority of prop- 

 erly orientated longitudinal sections,* and figures 10-13, '24, 

 43, and 54 in transverse sections. 



* The zocecium marked I, in figures 19 and 20, was at first thought to be 

 the ancestrula, since it communicates freely with the protoecium. A careful 

 study of the appearances possible in a series of longitudinal sections with 



