DEVELOPMENT Ot*' PtlASOPORA 359 



It is clear and structureless, with no evidence of fibers, laminae, or any 

 of the characteristic features of the walls of ephebastic zooecia. 



THE ANCESTRULA 



The ancestrula is a tubular zooecium of the familiar type seen in the 

 Cyclostomata. At first it is prone, lying along the surface of the sub- 

 stratum. Eising with gradual curvature from the substratum, it be- 

 comes straight, and at right angles to the base of the colony and extends 

 directly to the upper surface of the latter. Diaphragms and cysti- 

 phragms make their appearance very near the point of origin of the 

 ancestrula (figures 21, 23). It is evident that in the case of the Tre- 

 postomata only the lower portion of the long tabulated tube, seen in the 

 figures, can represent the true ancestrula, since this tube was undoubt- 

 edly occupied by a succession of superimposed buds. In this feature 

 consists, pa7- excellans, the prime characteristic of the Trepostomata. 

 The point of origin of the primary buds probably represents tlie distal 

 end of the ancestrula, sensu strictu, since, after the analogy of recent 

 Bryozoa, these buds should have arisen from the neck of the ancestrula. 

 The posterior wall, and possibly the entire wall of the ancestrula, has the 

 same peculiar texture as the wall of the protoecium (figures 1, 21, 22, 

 and 23). The ancestrula has about the same diameter as the protoecium 

 and is from one-half to two-thirds the diameter of the zooecia superim- 

 posed on it. Xo definite plane of demarcation between the ancestrula 

 and the protoecium exists in the Trepostomata, such as is to be seen in 

 FenesteUa and the Cyclostomata. Their relations are altogether more 

 intimate and primitive and seem to bear out my speculations in this 

 regard published in. my paper on the development of FenesteUa (7). 



THE PRIMARY BUDS 



The position of the buds adjacent to the ancestrula (figures 1-6, 9, 

 25-27, 36, 37) is such as to make it certain that two lateral buds and 

 one median bud arose from the latter. Two other buds {e and / in the 

 figures) also lie adjacent to the ancestrula, but by a careful inspection 

 of the figures it ^vill be seen that the proximal ends of these buds are not 

 in contact with the ancestrula, and that therefore they could not have 

 arisen from the latter. The arrangement of these primary buds is pre- 

 cisely the same as in recent Bryozoa (cf. figures 10 and 28, and figure 40 

 of my 1904 paper). ^ These two buds (e and /) are also directed pos- 



' In figure 10 (after Barrois) the left lateral bud is suppressed, only the median and 

 right lateral buds being present. Otherwise the plan of budding is the same as that 

 shown in figure 9. The buds surround the ancestrula somewhat more loosely than la 

 Prasopora (cf. figure 17). 



